IWC c641 Battery Replacement

Anyone know what the spec is for a replacement battery for the cal 631 as used in the IWC 3741 FLIEGER CHRONOGRAPH?
Thanks

Hi Bob
If my memory is correct it's Energizer 397 1,55 V battery.

Ordered. Thanks

Actually it was fitted with a RENATA 397 1.55v

But same idea!
Thanks again

Thanks for confirmation :o)