OK, I think I have it mostly figured out.

According to Volvo, there was never a factory installed square mount on any V70, euro or otherwise. However, there are aftermarket producers (Nordic) that make a cheap replacement for the mount that includes a square bushing rather than a round one. I had been assuming that my mount was original, but it seems likely now that it had been replaced once before. The hint is that 1 of the 4 bolts came out cross threaded. I thought it strange that a bolt from the factory would have been cross threaded, but it makes much more sense to believe that the PO put a new one on and cross threaded it himself.

I also called FCP Groton, and I have to say, they were no help at all. They didn't even understand the differences between the square and round replacements they had on their own site, and kept insisting that I could just buy a round poly bushing and it would fit. Also, they claim to sell a genuine Volvo mount that is square, and say that it is an updated design that is supposed to last longer, but in their OEM part number section, they just give the part number for what will be the round type from Volvo! It seems like they're being downright dishonest. If Volvo says they never produced a square style mount, how can they sell one as genuine Volvo?

So I believe I have a Nordic aftermarket replacement on my car, and can either replace it with another one, or go back to the round style. However, the phone experience I had with FCP has really soured my taste with them, so I may actually order the round style from the dealer and be done with it.