Any way to figure out/estimate speaker power rating?

[f13ndus]

Hey all,

I've got a pair of 15" Alnico speakers sitting around - 8ohm, Rola made, dated 1966. Is there any way I can estimate the power handling of these, based on magnet size, voice coil size, or anything else? They have fairly large magnets and I think 1.5" voice coils, might even be larger (I'm not with them right now or I'd measure them)

Just off the top of my head (or pulled out of somewhere else...) I'd guess maybe 20 watts? but I'm not sure. They seem to have a larger voice coil than my 1962 Jensen P12N, but magnet size is about the same... I wanted to put one in an open-backed cabinet as they sound really nice with my Gibson Gibsonette, I'm sure the 10ish watts from that won't be a problem; but I'm also building 5E3 head and would like to use the same 1x15" cabinet with that, but I'd rather not destroy the speaker.

I can get some pictures or exact measurements up if any of you think you could make a better guess?