I see this is slightly late, so I don't know if you've discovered an answer as yet. Also, I don't know if this will help much, but I have two 80s, a smooth, cross grain with a 1998 hallmark, and a Donegal 'Rocky', with a 2001 hallmark. Both have crisp 'P' stem stamps, as does a 1970 'Kildare' 260 cross-grain, oval shank, straight billiard and a mid 1940s smooth, straight, circular "Made in Ireland" stamped pot.
One of my 'system' full bents doesn't have a 'P' stamp, but it wouldn't, as the original stem was broken in a sailing accident. I lost 1973, 314 smooth bent "System" overboard in another sailing accident, after saving it in a fly fishing mishap, but it had a 'P'. They are/were all p-lips, by the way.
In many decades of inspecting, handling and smoking Peterson Pipes, I only remember two without a 'P' stamp, of one style or another, on the stem, however, I can't say that I was always particularly looking for them, unless it was to help date them.
Hope this helps even a wee bit.
-d-