The Franklin Porcelain marking is plain and simple. I am not sure what to make of the other marking above F.P. but perhaps someone on ICON could help.

Thanks for looking!

Update: It turns out the mark is an "I" (well, I knew that....) but the interesting thing is that some of the others that were found had an "L" above the F.P. marking. It's just an interesting shop marking that you don't see very often.

I spoke to Elton regarding the shop marking and he feels that it is not an "I" but is instead a "D." The more you look at the marking, you can make out the loop of a D and I feel he may be correct.

You can see more information in the April 1978 issue of Crown Jewels when it was reported that a U-310 Franklin Porcelain had the same "I" above the marking.