It's been a while since I'd been in this section and was a bit confused but read starting at S-5. Install Spring. There it talks about the rubber bushed shackles (also a description in S-2, i.e before and after 1957).
S-5 then says for U-shackles, go to S-6. So the LHT/RHT is all in S-6 for just the U-shackles - which included, "The left-hand threaded parts have been cancelled in the production of Models DJ-3A, CJ-3B, CJ-5 and CJ-6. All later production vehicles, up to early 1957, use all right-hand threaded parts."
Going out on a limb here, I'd guess that just like the LHT lug nuts, they figured it wasn't either necessary or cost effective.