After a couple of false starts, I've gotten an OH300 based prototype working. It's much better than the TCXO version. Of course, for $100 more, it had better be.

This result was with it being tested in the new extruded aluminum chassis. If there's a defect with that, it's that the power plug sits just a tiny bit too far back on the board, making the connection just a little touchy. I'm going to shove the connector backwards slightly on the board.

Boxing up the board adds another $50 to the price, but that includes all of the connectors and a power supply. Speaking of that, I found a particularly nice one at DigiKey. It's the one powering the OH300 for the above test.

If I have one concern, it's...

I don't know what those upward spikes are. They're not related to the GPS discipline, so far as I can tell. But they're only ~0.3 ppb in amplitude - not a whole lot to complain about. I also don't know for absolute certain that it's a measurement artifact or a real thing.