slash WHAT?

Slash what? (/?)
Okay, I was there in the 60's and the proud owner of an R69S. I lived in New York and was in the Butler and Smith offices many times.
There were the different models, R50, R60, R27, etc. and I never heard any reference in the spoken word, or in print, of a slash anything.
Then, in 1970, came the /5's, and it was some years later I heard references to the earles fork models as “slash 2's”. To make things more confusing, the early 50's models have been referred to as /3's.
It would make sense if the early 50's models were /3's, and the earles fork years as /4's, and the 70's models as /5's, but I guess that’s too easy.
So, how did this happen.

http://darryl.crafty-fox.com/motorcycles/bmwbikenames.htm

BMW started using slash numbers when they restarted motorcycle production after WWII. The first twin they built was based on the prewar R51. Although the new bike was very similar to the R51, BMW's engineers couldn't NOT fix a few things that they had learned during 6 years of accelerated R&D (eg, the war). So, the new bike was very much like the prewar R51, but updated. So, when they introduced it in 1950, they called it the R51/2.

But they had a lot more they could do, and immediately set out to upgrade that offering. The very next year, they had developed a completely new motor that was stronger, more powerful, with a better oiling system and better electrics. So, they introduced the R51/3. This motor design would continue for 19 years.

In 1956, BMW changed the frame design completely, went to a swing arm in back from the plunger frame. They also put the Earles fork on the bike. This was a large enough change that they changed the model numbers, from the R51/3, R67/3 and R68 to the R50, R60 and R69.

Unfortunately, the late 50s were a bad time for BMW, and in 1959 the company was nearly sold to Mercedes. There was little money to upgrade the motorcycle line. In 1961, BMW made a number of small changes, mostly not visible on the outside. Things like going to 3 ringed pistons, a vented final drive case, adding a gusset to the frame. The R69 got higher compression heads and more horsepower, and BMW upgraded its name to R69S. Internally, the R50 and R60 became the R50/2 and R60/2, but it wasn't publicized.

The word did leak out eventually, however, and especially when there were /5s, /6s and /7s, people began referring to the Earles fork bikes as /2s, as a sort of shorthand, which was much clearer than saying "the R60 before the R60/5".