Thanks for the tip, John,
From the size I deduce these are for Code 330 rail.
I've been sawing the smaller Code 250 ones in half to 'make' a similar product (with a washer of course).
I shall leave a few on the rails over the wet English winter, to see how their conductivity fares, but suspect I'll need to take them all off until there's an electric running session actually on the calendar. David