All early big-block Guzzis used the same F08 calipers on the front end. The F09 was used on the rear of some 1000SPs. If you can find F08s with the single 10 mm bleeder, those are the best as they usually have anodized alloy pistons instead chrome-plated steel ones. The 10 mm bleeder is also much less likely to twist off.
F08 calipers aren't what I'd call "rare", they were widely used on Guzzis, Ducatis, Laverdas and BMWs. The F09 is sort of rare.
If you're looking at the same ebay calipers as I am, then $250 for a pair that might need to be rebuilt is a bit high IMO.