I have isolated the source of the spurs.
The clue was something I just read on the Keysight forums - that 25.5kHz was the sync frequency used for the external monitor. Aha, that is also the offset of these spurs from the CW.
Initially I thought my routing of the external monitor cables was the issue. Playing with that didn't help.
Then I tried disconnecting the CRT and blind keying into CW. Big improvement in all except the 100kHz spur which gets worse for some reason.
That spur has its own tuning pot but I couldn't reduce it any further.
The key question is does this affect the device? I have noticed some very occasional glitches in the CW signal, which makes me wonder if the spurs are high enough to confuse the phase lock. Apart from that everything seems comfortably better than spec.
85044A T/R Test set has arrived. Its ex Colorado Uni and by the look of the dings in it, it has taken some punishment. I'm one 7mm to SMA adapter and one N to SMA short
so can't fully test it. However I tested each channel (R and A) individually and it seems to work OK.
Pulled the cover off it - it has little plastic zip slider things, no screws. Opened up the directional bridge and wow, it is a thing of beauty inside. Wish I had a better camera.
Top gold box is coupler/bridge, bottom silver box is variable attenuator. Connections on right are - top bias from VNA, middle top A to VNA, middle bott R to VNA, bottom RF from VNA
Inside the gold box.
Old repair
RF input comes from the right via the choke, bottom is test port, left goes to reflected A output to VNA, top goes to termination resistor.
Tiny Termination resistor at the end of the line from the Test port.
13dB Compensation, to cable strangely routed the long way around the front of the case, goes to R output
Splitter - top exits to variable attenuator, botton to fixed 13dB complensation
Pin coming in on centre right is RF input from variable attenuator. Heads left via the choke to the coupler Resoldered cap visible. Lower right is bias input.
Edit: Found a schematic for the 85044A Test Set