Understanding a Hardware Description Language, circa 1965
I can't answer how to do this in Verilog, but I think it's safe to say that C/K is being used here to control the propagation of a signal through a latch.

I think the original intent was very obviously for C/K to control a latch here.  The implication then is that this HDL was made for sequential circuits and that it performed all assignments at the same time (i.e. it was 'clocked').  So new values of RAcc and RxAcc are calculated based on the previous values, then the new values are written to RAcc and RxAcc both at the same moment.  The problem of either C or K being zero then doesn't occur.

To put it another way, this HDL appears to use a model where variable assignment means "deposit this value into this variable at the end of the current evaluation cycle", while variable reference/evaluation means "retrieve the value this variable had at the start of the current evaluation cycle".

To put it yet another way, variables in this HDL appear to have their own virtual latches clocked by the simulator clock.  New values are fed into all variables at the same time (call them registers if you like), but the variables don't 'accept' the new values until the end of the simulation cycle.  Having this register-like behavior built into the language makes it very easy to represent latches using this C/K idiom.

This register-like behavior for RAcc/RxAcc may be triggered by how they are declared earlier in the file, or this may be default behavior.  I can't tell from the snippet shown.  If it's default behavior then this language may have a special syntax that is used for combinatorial circuits when a value is desired to be stored/retrieved instantly.

Also note: this language may not have had an actual simulator program to go with it -- the "simulator" it was written for may have been the human mind.  If this was intended to be a human language rather than a computer language then the rules of the language don't need to be rigidly specified -- you can rely on context and human understanding to make the meaning clear.