I'm sure all the experts would already have this or something similar. But just in case there are people reading this who may not have it, I'll post the link here.
I found this after going to change the battery in a beautiful Classique pocket watch that the lovely Mrs Rock had bought for me some time ago, engraved with my initials, for an anniversary. Inside, I found a Renata 371 battery, and among my collection of battery cards, there was nothing by that number.
So I consulted Professor Google, and found this - very useful. I put an SR920SW in and it fitted perfectly and works perfectly. Good to get the old girl going again! The watch, that is.... ;)
http://tuxgraphics.org/~guido/button-cell-battery-cross-reference/
And of course, there is this, looking at it from a different viewpoint:
http://en.wikipedia.org/wiki/List_of_battery_sizes