Good info on the twist in the shackles there. To clarify the LH/RH shackles, that's correct for the M38's (& earlier jeeps). According to the ORD 9, the M38A1 had "retaining plates" fitted to the outside of the spring, like this (item Q):
Interestingly, page 293 of TM9-8014 shows the M38A1 shackles being the same as the earlier jeeps (no retaining plates), with page 295 showing the retaining plate configuration, supposedly only for the M170.
What gives here? My M38 came with a set of what I believe to be M38A1 spring shackles with the retaining plates on (in the box o' spares). I would think very few (if any) M170's made it to the UK's sunny shores; did Willys use the M170 shackle design on later production M38A1's??
Now, if you're using the earlier shackles (i.e. two of the shackles have both LH & RH threads), then the LH threads go through the spring eye. That should tell you which way round your shackles need to go, in relation to the twist.