The original name of the company was Knapp and Peterson. Eventually, it became just Peterson, but the K&P trademark was already establish and I believe has been in continuous use. Just from personal observation, it seems that all silver is stamped K&P
Please don't take my comment about silver too seriously. It is just my own observation. I could be completely wrong. And if I am right, there are always exceptions to the general rule. It is not uncommon for inconsistent stamping from Peterson. For example, I have a Peterson Killarney pipe stamped "MADE IN IRELAND", which would "date" it to the early 1900's. However, I purchased it new in the 1990's.Interesting point that the silver is stamped K&P. I have a hallmarked 1950 K&P, but a 2015 Christmas pipe that just says 2015, I guess that's just polished metal and not silver?