Gerry Sweeney wrote a post on his blog detailing his DIY HP/Agilent 53131A 010 high stability timebase option:
I decided to make a clone 010 option board for my counter using a second-hand OCXO bought from e-bay. I designed a PCB to get a professional finish as well as a reliable upgrade for my counter. The main goal was to make an option board that just like the original could be automatically calibrated using the internal software and front panel controls so I had to use the same DAC chip (which is now obsolete) and basic topology of the original option board to make it work.
Check out the video after the break.