Honestly, my thoughts on this are how many of them could possibly be hacks made just for MAME?
If that's not what they are then they MUST have been made to run in a cab somewhere.
I was around when MAME started and for some reason it just doesn't make sense to me that there were people talented enough to make such a bootleg for MAME for no other reason than just to do it and they are still unknown.
I'm sure there are a few and I'm sure the person that made it is proud and wants to be known for it.