Here's a photo of a Simplex mounting bolt offered by a French ebay seller. I've also attached a closeup of my damaged bolt from the same angle--the seller's photo is at the top; mine is at the bottom. The two seem to be pretty much identical, don't they?

Of course, there are so many ways for the new bolt not to fit by a few thousandths. On the other hand, it would have made sense for Simplex to figure their parts in such a way that one mounting bolt would have fit many different derailleurs. On the third hand, experience shows that "making sense" was never a high priority for Simplex.

The seller describes the bolt as a #3874 bolt for Prestige. That could mean almost anything. If I could find an exploded view of the long-cage SX610 that specified the number of the bolt I'd be in good shape; ditto if I could convince the seller to get out a micrometer and measure the various lengths and diameters of the bolt he has. Or I could just order it and hope for the best.

Does anyone have any light to shed on this question before I vaporize a perfectly good $20 bill?