Just a quick update, replaced the original Trimble 65256 OCXO with a China sourced used 65256, the original was at the VFC DAC upper limit of 4.9v. The replacement is sitting at ~2.3v so seems there is plenty of life in the replacement part.
One interesting thing I noticed, even though the 65256 is spec'd for 12v Vcc its being fed a regulated 5v. There is a 0 ohm resistor that can be moved to feed it 12v from the input power, but I noticed no difference in performance of the OCXO running from either 5 or 12v other than initial oven warm up power. Perhaps the thinking here is a regulated 5v to the oven will provide slightly more stable oven regulation, so I left it as such.