Thanks for looking this up. I can't say I agree with the C4 designation when I measured the handstamp at 2.5 mm and 2.0 mm respectively, but as you stated earlier there could be certain varieties not recorded.
I tried looking it up on the internet and came up with the same possibility. Not sure if "UGI" as a company name was around in 1941 though. It might be something to keep for a rainy day to look up in further detail. Right now I have too many other dateds that are more easily identifiable (I think). (I'll post some of the more interesting ones as I start sorting through them.)
I know nothing here is going to make me rich, but it certainly is an interesting sub-topic to precancel stamp collecting and every once in awhile an apparent new find or unlisted variety comes to light, which is always fun to uncover.
I was just wondering are you sure that the SKF for the 2 month period in 1955 is Smith Kline & French? Could the SKF refer to SKF Industries a bearing and seal manufacturer located in Philadelphia? They also have a division in my town Elgin Illinois.