The 4" is a -4. I bought it brand new in 1978...still have the box and invoice. The 2.5" is a -6 (1982). I bought it at a gun show in 2007. Both are great shooters and they' re my favorite guns.
You have good taste! Glad you saved the original stocks. Too often they were replaced with the Goodyears and then lost. The 2 1/2" 19-4 I inherited had Pachmayr stocks but I was able to find a nice set of un-numbered magnas at a gun show. Best $3 I spent that day.