Another product from Frequency Electronics is the FE-405B. It's a really good DCOCXO (an OCXO that's digitally controllable). It's also available on eBay, though perhaps not quite as widely as the FE-5680A. Tom Van Baak has an excellent overview page on it.

From a hardware perspective, the FE-5680A GPS discipline boards are compatible with the FE-405B. The firmware does need adjusting, however. The FE-405B output is 15 MHz rather than 10 MHz. It also doesn't have an !OSC_RDY output on pin 3, so the firmware needs to ignore that (it also lacks a PPS output, but all the board does with that is take it to an optional pin). It also has a different EFC gain.

At first, I believed that the 405B would be a niche application of the board, but the reaction has been surprising - people seem to want both.

I can understand the attraction. The 405B has excellent short and medium term stability (under 1E-12 out to tau 100s), and that's what really matters for a GPSDO. So I'm going to try and get one and tune the firmware up for it.

The 15 MHz output is awkward, though. To use it as a reference for my 53220A, I'd need to divide it by 3 to get 5 MHz.