Ok, compared the RG-58 Tektronix cable to an RG-400 I built of similar length.
At 200Mhz and 4Vpp (x1 amplitude) we have ....
The OEM Tektronix cable clocked in at 0.20 dBm low.
DIY RG-400 cable was only 0.01 dBm low!!
Now B4 you guys roast me over the Boonton 4210 microwattmeter with the 4210-4B sensor....
I make no claims, infer from the results what you will (expect 1.5% accuracy from the Boonton).
But...I feel pleased that the SG 503 combined with a RG-400 cable and the Boonton show what could be a nearly perfect output.
I must mention that I recently serviced the SG 503 using liquid de-oxit on the rotary frequency selector contacts and polished the brass nut and seat contact point on the variable tuning capacitor.