I have a OS SX50 in front of me here and I measured the C-C distance as close as I could with the Mitutoyo Digital calipers and it was 37mm or 1.46".

I was reading in the Tower stuff on the net and You cain't believe a word they say about C-C dimensions. They have the Macs for the ST and the one for the OS at 36mm. Then they list a OS 46 muffler at 36.5mm and the Tower 46 at 37mm.

Maybe you can measure your 51?

Enjoy,

Jim