When were the round Underwood "rebuild" bolts made?

I have a new (bare) round bolt wrapped in cloth and heavy black grease. When I bought it, the seller said it was his last one and it was GM. After joining the forum and seeing all the discussions on bolt markings, I decided to pull back the wrap to check the markings. It has the .u. beneath the left lug, so it's Underwood and not GM. It also has the gas escape hole on the bottom. I wondered if these were from the late 40's and couldn't find any information in the books I have, other than "post war." Last night I did some searching on the Internet and found 'carbine forum' posts showing that most were made in 1952 or 1953. Chesnut Ridge has complete Underwood bolts with packaging that looks like 1956 on the label. Some questions: were the Underwood "rebuild" bolts all from the 50's, and was there such a thing as a GM "rebuild" bolt with the gas escape hole? If so, how was it marked?

Thanks! - Bob