I was able to confirm the Cohu 303B electronics were significantly different to the 324, and the 355 models (from their service manuals), so if anyone has a scan of the 303B schematic I'd appreciate a PM to assist some careful rejuvenation of the e-caps and confirming the all-valve electronic sections are operating as intended.
This 303B has an ovenised reference 7817039-001, and uses a Stevens-Arnold CH-801 DC-AC chopper. The 7-decade feedback resistor network has the 12 largest resistors using their own CONELCO 0370L-2 trimpot. The 1400V raw HVdc uses a large oil-can cap, rather than the latter models that went for a series string of 450V e-caps.
It's interesting that the 303B, from circa 1964, was bested at that time by the 304 model.