I started to look at the Thief for Liber Zero, but at first didn't notice any rolls mentioned for the climbing skill. I was beginning to think that LBB thieves, unlike later versions, were just give a flat auto-success, which would have been nice. But I finally found the necessary paragraph. And, unfortunately, it's a percentile-based roll in increments of 1%. I've sworn to avoid them in this clone, so I may have to be tricky to create a work-alike clone version.
Thieves can climb sheer surfaces without equipment, and non-thieves can climb using appropriate gear or handholds cut into the surface. They climb at 1/3rd their normal movement rate. If climbing more than 10 feet, make an Avoid Danger roll to avoid slipping on a 6 or higher. If the climb takes more than 1 turn, roll every turn.
Thieves, and Thieves only, get a saving throw to catch themselves as they slip. Treat this as a Target 20 roll, adding the Thief's level+3 to the d20 roll. A result of 20 or better means the Thief doesn't slip.
What this means is that there's a 16.67% chance of slipping, but a Thief who slips only falls 80% of the time, which gives an adjusted chance of 13.34%, close to the 13% value in the LBBs. Each added level lowers the chance by 0.83%, again close enough to the 1% standard to make it usable. As compensation for the slightly-lower chance of success, they have automatic success for climbs of 10 feet or less, or for using climbing aids. The Thief can now safely descend into 10-foot pits as many times as desired to see if any previous victims have left treasure.