The RT68 was part of the GRC-3 system which also used the RT66 or 67 and the RT70.
I haven't seen any info (photos or manual) on it's use in the M37 however the military did evaluate them. There is a file in the Library & Archives (HQS-8812-7-5 ) where the US system was compared to the C42 & C45 sets.
It's an open file (not restricted) so it may give a clue.
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