I started this thread originally because I was accidentally sent the wrong lower parts kit and wanted to be able to tell which one I actually had. After having both a PTAC and a PSA Classic LPK installed in the same lower, here are my thoughts.
I ordered one of the lightweight premium uppers with what was supposed to be a PSA Classic build kit about a month ago, but it arrived with a Blackhawk stock/grip and a parts kit in a plain plastic bag (all of the other PSA Classic LPKs that I've seen have been in either the white cardboard box and/or the plastic bag with the blue PSA logo on it). I suspected that since I was sent the Blackhawk furniture, I likely had a PTAC LPK as well. The most noticeable quirk was that the trigger was very gritty on initial takeup even though the contact surfaces on both the trigger & hammer were polished.
Around that same time, a co-worker of mine received a PSA Classic MOE kit to put in his new lower. I borrowed his finished lower and did some direct comparisons between several of the components in our two lowers:
1) First of all and most importantly, his trigger felt just absolutely fantastic compared to mine. This was a deal breaker for me because the feel of the PSA classic trigger is why I wanted the Classic LPK in the first place.
2) Takedown & pivot pins appeared lighter in color on mine
3) Trigger & hammer springs appear to be slightly thinner or have a different finish on the PTAC kit
4) My buddies selector switch has a nice crisp positive engagement; the PTAC feels a bit sloppy in comparison
I contacted PSA customer service through this sight; they were fantastic to work with and I'm extremely pleased with their response and the outcome. Based on the fact that I had the Blackhawk furniture, they figured I likely had the PTAC LPK instead of the PSA Classic. They worked out an exchange on the LPKs with me. I just now got the replacement PSA Classic kit installed and I'm 100% thrilled with it. Trigger is perfect & the selector feels just like the one on my buddy's lower.
So in summary, the PTAC LPK is probably fine but I had felt a few PSA Classic triggers before...and the PTAC trigger just wasn't there, IMO.