My Beeman R10 which is the same as the 85 Mk1 has a stroke of 89mm vs 85 for the R9 or later 95. The piston is also a bit lighter with the cutout at 260 grams vs a bit over 270 for my R9. In full power tune over here in the states, the R10 makes 14+ ft/lb easily with only .120 wire spring and has a more pleasant shot cycle although requiring good technique and follow through for best accuracy. A lazy feeling shot cycle.
I was a bit surprised at the difference there was with only 4mm longer stroke. My HW77K on the other hand is 4mm less stroke at 81mm. A different animal altogether though with its short TP and the sliding cylinder. Much quicker shot cycle. Easy to hit with, but not as pleasant.