Dave's unit has a ser. of 2911A65826, falling into the post 2619A37795 units with a different design to the pre, and part of the serial could be the date code, the 1st, 2nd, 3rd and 4th digit, SN : 2911A66xxx - Means : (29+60 = 89) = 1989 , Week 11 , A(merica) (info from bingo600)
Also, if you don't know the serial no, but "TERMINALS" shows up instead of "TERM" for the front-rear switch front panel, it's a late unit (for those aiming to purchase a later model).