I acquired my this Mar-Bar rig from Bill Liebman in 2017. It is not a commercial product.
The Liebman Jumar Mar-Bar rig is 191 mm. tall, 304 mm. wide, 74 mm. thick, and weighs 617 g. It consists of an old Jumar ascender attached to a rectangular aluminum alloy foot plate with a milled aluminum block and wo 1/4-20UNC hex cap screws and hex nuts. The plate is 305 mm. (12 in.) long, 47.8 mm. (1.9 in.) wide, and 6.8 mm. (0.27 in.) thick. It has a 9 mm. hole near each corner, and a 12.7 mm. (1/2-inch) hole near each end. A strand of 9 mm. BlueWater III rope knotted below the plate passes through one hole, through a green hose and the upper ascender opening, and down through the other hole where it is knotted again. The milled aluminum block is 52 mm. long, 22 mm. high, and 12.2 mm. thick with a 16.7 by 6.8 mm. channel cut for the lower jumar strap to pass through.
The top of the bar is engraved with "ORIGINAL Mother Fucker Stand."
Mar-bars are used in the inchworm climbing system, a sit-stand system that is particularly easy to teach, and one that worked rather well on long, free drops. Cavers rarely use the inchworm any more, but recently some arborists have adopted it, and commercial Mar-bars such as the Buckingham are now available.
Old Mar-bar rigs were homemade, and often crude; for example, see the one that Mike Walsh gave me. Bill Liebman's is an exception that is carefully made.
One thing that I do not like about Bill's Mar-bar is that the weight is transferred to the lower ascender strap, a known weak point on the early Jumars. A much better design would have beeen to have the weight supported by the thicker back portion of the lower handle.
To improve efficiency, one should ensure that the rope loops fit tightly.