The RA1792
Page 3

I am not even going to try to describe the operation of the 1st LO synthesiser other than to say that ultimately there are two distinct versions; MK3 & MK4 … the latter of which is based around an LSI chip (the one with the gold top). In both cases the reference frequency comes from the 2nd LO synthesiser board … 1MHz for the MK3, 20MHz for the MK4. The LO o/p is between 40.605MHz and 70.445MHz.

The 1st LO Synthesiser
MK4 LSI (covers removed)

Above, we have two other 1st LO boards. One of these works if it is addressed as a MK3. The other is faulty. The differences between the two are minimal. It is possible that these are essentially MK2s, although I have no proof.