I acquired my Buckingham Mar-Bar from Tetyana Zurigat in 2007.
The Buckingham Mar-Bar starts with a pair of large CMI UltrAscenders (my version C). A 300 mm. rod goes through the bottom rear attachment holes, and a 140 mm. rod passes through the top holes. A 150 mm. piece of flexible tubing forms the left side of the rig (right side in the photo), and a shorter piece stiffens the opposite side, above the buckle. A piece of 25 mm. chain-stitched webbing starts at a sewn in buckle on the left, passes through a rubber ring and then through a 145 mm. long piece of 32 mm., 4.7 mm. wall PVC tubing and over the lower rod, through the ascender holes and another piece of PVC tubing, over the right side flex tubing, over the upper rod and through the upper ascender holes, and then down over the final flex tubing piece to the buckle. Finally, two more pieces of course webbing cover the upper horizontal section of the webbing frame.
The Mar-Bar (short for Jumar-Bar) was first conceived as part of the Inchworm climbing system, an interesting historical innovation that allowed for relatively efficient, hands-free, sit-stand climbing, even for unskilled climbers. The system all but disappeared from caving scenes in the 1970s, so it is interesting to see the Mar-Bar idea revived by arborists, even if they do not use it in an inchworm system.