Didn't call you a moron, FYI. I used obtuse in the "slow to understand", and "rejecting alternatives without cause" meaning.
I've had a 5500 since they were new. Was, in fact, my second audio purchase. I know the hole in the chassis is large, but the washer should bridge that. Otherwise, a piece of sheet metal could be fashioned to bridge the gap.
Bottom line is that original parts like this have been unavailable for decades, and a bit of determination and ingenuity is often required to find alternative solutions. If you have found one, by all means, share with the group.