Hi!
31115 HF is set at 10k, and has less frequency options in the mid-range. Not older, same time, just a cheaper option than 31114, which has 5 frequencies in the HF(If I remember correctly.). 3114 is a darker blue-grey, colour on the knobs(well funky!), and with, I believe, different input transformers. Sound only slightly different to me! I used to work on a board with a mixture of the above. bigger difference is whether amps are discrete or not in my book.
Blah!
Andy P