I always thought I had a 1988 Triathlon but it was a guess based on these facts with only Jim's catalogs from 1987 and 1989 to use as a guide.

Pre-1989 characteristics

1. The paint and decals look identical to the 1987 paint and decals
2. The frame and fork are Super Vitus 980 with lugs. The 1989 is Reynolds 501 without lugs.

Post-1987 characteristics

1. The brakes levers have the hidden cables like the 1989
2. The derailleurs and brakes are all Shimano
3. It is indexed shifting on the rear with friction on the front derailleur

The strange part is that the s/n looks like it begins B6... but it does not resemble a 1986 Triathon much. Interesting that the original posters bike may be 1988 but is Reynolds 501 without lugs. I bought this bike from the original owner for his asking price of $75. He said he replaced the pedals and might have replaced the freewheel but the freewheel is a Sachs unit so maybe he did not. I replaced the seat with the leather Perjohn NOS I picked up a few months ago.