Dan Watson documented on his blog an OCXO upgrade board he designed for the Fluke/Philips PM66xx line of frequency counters:
My counter came with the standard XO timebase option, which has fairly poor specs for stability and drift. It is difficult to trim precisely with the single-turn trimmer capacitor on the board. For most testing in my lab I use an external reference from a GPSDO, but it is still nice to have an accurate timebase available in the counter if I need to take it somewhere and do testing away from the bench.
Previously I posted about an OCXO upgrade I made for my Racal-Dana 1992. The fun of designing a similar upgrade for the Philips counter was one of my motivations for purchasing it. My upgrade board is roughly equivalent to the original PM9691 OCXO module, and it should be compatible with any Fluke/Philips counter that is capable of using that option.