Hi mate, both my workshop manuals show a mix of round and square, some even mismatched , yours is the early 60 series, in 1987 they done an upgrade , mine is one of those , usually easy to pick by the headlights, they had a complete new chassis, springs were changed to a long travel leaf (whatever that entails, maybe length)
I've interchanged between early and late with a few problems, I have swapped out springs, diffs, shafts and transfer cases and have come across bolts patterns not matching, and different size locating pins on springs, to remedy the bolt pattern problem I have either changed the end of the shaft or the coupling plate on the transfer or diff, I seem to remember having hassles with length of tail shaft at some stage, but not certain, I think engine position may have even changed with the upgrade.
Ok, it gets tricky because on both types of 60 series you have variations such as 2f, 3f, 2h, 12ht and a 2h upgrade, which is basically the same as the 12ht minus turbo.
Then there was 4speed and 5speed manual, and an auto.
There was different spring set ups with rubber and crush tubes cushioning the union between spring and diff on the Sahara , the barn door pov model like mine just kept basic steel on steel, although mine now has the Sahara set up because that's the diff I have.
Then after all that there was the standard open type diff or the limited slip diff.
And...there was lots of imports , these were heaps different with 24volt systems, different interior, park bench winch bar with huge pto set up.
There were lots of options in both type of 60 series (pre1987, post 1987) it has had me stumped over the years why did they change bolt patterns??????
By the way my rear tail shaft is close to yours in length, and my plates are all square but have been swapped.
I did spend quite some time googling to no avail, maybe a Toyota mechanic who has worked on them in past years could enlighten us, my old girl is so mismatched from up to five different donor vehicles whatever I have doesn't help , it is a bit of a Frankenstein.
Both these pics are in the same "to remove and install" instructions ???? Both round and square ????
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Auto box and transfer with round coupling
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Manual 5speed and transfer square coupling
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there is no mention of coupling differences or of shaft length differences in the manuals,
so there you go absolutely complex and bloody confusing, but gee I would like to know the exact specs and parts used in each model/ variation.