Hi! About that comparison between 53T6 and PRS-1:
I would assume that there are indeed some differences. Why?
Well the question should be: why does the Soviets skip ABM-2 and were marching towards ABM-3/4?
It was already mentioned that the performance of 5Ya27 was somewhat disappointing although the similarities of this design in respect to the successor 51T6 are distinctive.
This could be the same with 5Ya26 (if ever realized) in respect to 53T6.
Anyhow, I would not see a reason to delay the deployment of an already existing system (PRS-1) if the intended functionality is given with that design. Or simply spoken: they could have a Gazelle 10years before if 5Ya26 would work. And if it was only to gain some experience than an improved follow up (although with many similarities in layout) would be the consequence (53T6).