After going through my picture collection ignoring restored examples as well as rebodies/recontructed cars and focusing just on clear pictures from cars that appear to be undisturbed examples I found the vast majority of examples had a washer with each bolt that attached the front bumper brackets to the frame rail. There was one example #176x with out washers and two examples with large flat washers I don't believe are original and might be related to early damage or replacement for some unknown reason. Washer found were small dia thick washer as shown in some of the examples below . On cars where I had available pictures of both sides the hardware match from side to side and all four bolts or the example was not considered
Going to limit the pictures to a handful rather than showing of them unless someone feels that is needed
First example shows a regular San Jose 67 Mustang, its bracket and retaining bolts with washers and below 67 Shelby #173x
67 Shelbys #110x & a different #173x NOTE: This is one of the only ones were washers don't appear to be there. Did the worker loose the San Jose hardware with washer or did a worker at San Jose just make a mistake and in turn produced the results. Either way it appears not to be typical or normal but had to share
67 Shelbys #145x
67 Shelbys #110x & a different #173x
67 Shelbys #184x & #250x
And lastly the washer, like the ones we find on 67 San Jose Mustangs were likely installed at San Jose as bumper brackets and bumpers were installed there right along side of the Mustangs and deliver that way to the Shelby plant. No reason for the workers to take the time to remove the washers IMHO while they swapped the support arms just easier and cheaper to reuse them.