Here's an email from Rick, posted with his permission:

I didn��t forget about you!!! I��ve finally gotten around to doing a thorough look-see into the wave washer issue. I��m not able to come to a conclusive statement one way or the other, it looks like one of those deals where Ford engineers were changing things around for questionable reasons. Whether or not it actually translated to a change of procedure at the assembly line is debatable. I think somebody like Bob Perkins or Ed Meyer would have the ��real life�� answer. Anyhow, here is my body of evidence, in no particular order��

The Ford Parts Illustrations Manual (different than the Osborn Factory Assembly Manual, this is the text that dealers used in context with the parts text) shows a blurry picture of a washer going between the bumper and the bracket, and identifies it as ��17A924��, which is the base # for a wave washer. The illustration is attributed to both 1969 and 1970 model years.
The Osborn manual for 1969 specifies 17A924 wave washers be used, 2-per bracket. As-such, the AMK kit for 1969 includes wave washers..

The Osborn manual for 1970 does not include the wave washer. In fact, you can see in the detail column on the RH of the page that # 2 has a part ��D��, and then skips directly to part ��F��, skipping the letter ��E��. In the ��69 manual, it��s the letter ��E�� that specifies the wave washer. Also of note, the 1970 manual specifies a different bumper bolt for 1970 (383464-S100), rather than 1969 (C4ZB-17758). The bolts in the AMK kits differ accordingly, the 1970 bolt (383464-S100) being less ��domed�� and more flat on the head, and also an 1/8th of an inch longer. But there is NO difference in the square-shaped shoulder underneath the head, which puzzles the **** out of me. It��s the height of that shoulder that necessitates the use of the wave washer. Without a wave washer acting as a filler between the bumper and the bracket, that square shoulder butts-up directly to the bracket (due to it extending far beyond the thickness of the bumper), which isn��t a very optimal ��flush�� mount. Why would the engineers do away with the wave washer in 1970, and change bumper bolts, without remedying the issue that necessitated the wave washers being used in the 1st place??

I haven��t removed the rear bumper from my ��70 Mach, but looking up from underneath the car, through the gap between the bumper and the valance, you can see wave washers peeking out between the brackets and the bumper on both sides, in the upper positions�� Were they put in there by the owner at some point? The rear of the car does have a little bit of ��touched-up�� damage, where it looks as though the bumper got lightly tapped, and possibly replaced. So the originality of my mounting hardware is in question.

The rear bumper on my ��70 Deluxe Sport Roof 428, which was restored to Thoroughbred spec, has wave washers..

So that��s all I��ve got. The Osborn Manuals, and subsequently AMK (who probably based his kits upon the Osborn manuals), show different hardware used between 1969 and 1970. But did it actually happen on the assembly line? Wouldn��t a line worker doing bumper arm attaching say, ��hey, if I don��t use a wave washer, then this square shoulder butts directly against a round hole on the bracket, which is no good..��. Would they continue inserting wave washers, as they probably had a bin full of millions of them at arm��s reach? I don��t know. But I do know they make a more sensible install, from a torqued assembly perspective. I would use the correct flatter-head AMK hardware kit, and slip the wave washers in behind, where no judge will be looking anyhow. And even if they do look, I have serious reservations as to whether the assembly plants didn��t continue to use them despite what the assembly manual spec��d. Also, it��s a subject that you should post on your site, and I think I��ll post on the concours thread at VMF, to see if there��s more input from guys who��ve disassembled enough ��known virgin�� 70��s to have an authoritative opinion.

Rick

What I can add to this is that the holes in the bumper mounts are large enough that the square shanks fit within them easily enough so they clear. The bolts then just use the square holes on the bumper to resist rotating as they are being torqued.

I agree that they 'should' be used, even if only to prevent dimpling of the bumper.

If anyone has any experience pulling the rear bumpers off original '70s and you can remember if they had the wave washers or not we'd love to hear about it.

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oddly obsessed with big scoops on little Mustangs

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I am curious. What evidence drove you to your conclusion? I am not questioning your conclusion. Just curious.

1. First off, without a wave washer creating a crush-zone between the shoulder of bolt, the bracket, and the bumper, you can't really tighten the bumper to the brackets "properly". That's just plain observational geometry. That's why I started digging into this, as I couldn't imagine "why" Ford would remove the washers from the 1970 Assembly Manual

2. Second, the Ford Parts & Service Illustrations Manual DOES specify that the wave washer applies to both 1969 AND 1970.

3. Third, is just plain old people, with original unrestored cars, that have confirmed that they have wave washers between the rear bumper and brackets on their '70. I got a guy on VMF who attested to that fact on his unrestored virgin car, buening on here, my own 4,900 original-mile virgin 1970 Mach has wave washers, and Bob Perkins has owned more ultra-ultra-low-mile 1970 Mustangs than I can count. His 1970 Boss 302 has, what, 1,700 original miles?? It's a beautiful piece, fully documented from new. It has wave washers... Bob Perkins, who is arguably the most prominent authority on 69-70 Mustang originality, was my final silver-bullet. He says that there is no question that wave washers were used in '70, and the manual is dead wrong.

So basically, our evidence is that it would be mechanically improper to not use wave washers (given the square shoulder of the bolt being far taller than the thickness of the bumper steel), some Ford books say that they were used, the Ford Assembly Manuals are not always accurate (published prior to actual production, and prone to change/adjustment/omission), and every virgin car we've checked and every authority we've asked has confirmed that the assembly manual's ommission is pure hooey, misleading.