"What relation to sizes such as "5L" have to sizes such as "B"? When I bought a belt for the drive on my lathe, I asked for a 5L, as informed by someone on this site. The counterman said they don't carry 5L, but a "B" section belt is the same thing. Works fine in the lathe, so I assume that's true. What's the deal with this?"
4L is equivalent to A/AX (with exceptions), and
5L is equivalent to B/BX (with exceptions), etcetera.
However, the L suffix indicates light duty, and not matched as to length, whereas the A/B prefix, etcetera, indicates heavy duty, and matched as to length, according to the RMA (Rubber Manufacturer's Association) standards for "ganged" belts.
(There is no heavy duty belt which corresponds to a 3L, which is quite popular in early Hardinge Brothers products).
However, if you do need matched lengths of 3L, 4L or 5L belts, and RMA belts (A/AX, B/BX) are not an option, be aware that belts are cut from a master belt, and the master belt number, and the position from within that master belt which an L belt was cut are encoded on the edge of the belt, near the manufacturer's logo.
If you need three matched length 3L belts, as for a Hardinge TL or TM/UM, you may notice that matched sets of Browning "Matchmaker" belts, the HB OEM belt, are no longer available.
However, by carefully selecting from retail 3L belts of the same manufacturer and "trade size", and matching the master belt and position codes, you can obtain a pretty well matched set.