dabac

I can't tell 25.4 from 31.8 from that murky pic. But it's a big enough difference that YOU should be able to measure it easily enough by putting a ruler across the broken end.

Or how about this:
Have you got a credit card? A credit card is about 54 mm wide. If your bar is less than half the width of a credit card across at the clamp, you need a 25.4. If it's more than half a credit card at the middle you need a 31.8.