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I acquired my John Blum, Version A box from Inner Mountain Outfitters at the 1988 Old Timers Reunion.
My John Blum, Version A is 77 mm. long, 127 mm. wide, 44 mm. high, and weighs 256 g. It is a double channel chest box with a cast aluminum frame. The back plate has 51 mm. tall vertical slots for attaching the box to a chest harness. The roller assembly pivots at one end, and is held by a quick-release pin at the other. The rollers are nylon, with a larger nylon bushing to keep the rope and foot sling separated.
The frame is stamped "JB1A"
One thing I don't like about the Blum box is that it opens both sides at once. There is no way to open the foot sling side without releasing the main line and vice versa. For this reason, I never used this box for regular caving.
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I acquired my John Blum, Version B from Bob & Bob at the 1993 NSS Convention.
My John Blum, Version B is 64 mm. long, 175 mm. wide, 43 mm. high, and weighs 327 g. It is a double channel chest box with a cast aluminum frame. There is a 42 mm. tall trapezoidal opening on each side for for attaching the box to a chest harness. The roller assembly is similar to Version A's but the Version B rollers are brass.
The frame is stamped "JB2-9322."
Version B has a larger frame and brass rollers. Even with the large cutout, Version B is 28% heavier than Version A. As far as operation goes, it is similar to version A.
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