Welcome to the new and improved Proflex / K2 Tech Forum!
I'd say it's a Small 656.I'd suggest that the decals are not original (as fitted) should be "PRO~FLEX" on the downtube and "656" on the top tube.i.e. pre K2 days or at least before they were badging them.I don't believe that "Animal" or "Beast" were ever used for this model.If the rear shock works OK, then I expect it'll make a nice "little" bike!Good Luck Col.
I would put it all the way back to a 655 with the chromoly swing arm and no down tube gusset. Just guessing. I believe that bike came with elastomers in the back too.
The Attack/Reptile frame drawing looks 99% the same except the frame on my proflex is different where the rear shock mounts to the seat tube. On my frame its a triangular piece thats welded to the frame as opposed to the round stem that protrudes on the Attack