[Test-Equipment] Phase lock on military version of HP 8640B generator?

[Mike Czuhajewski]

I have one of the big, yellow AN/USM-323 signal generators, also known as
the HP 8640B with option 323. I read that the commercial version has a front
panel control that allows phase locking the output to an internal reference
oscillator for stability. The question is how this relates to the yellow
military version. In some places I've read that it does not have the
external control and that the phase lock is on all the time, and elsewhere I
read that it does not even have the phase lock. I don't know which is
correct.

My unit drifts slowly for several hours before finally settling in. Is that
normal for this unit, not having the phase lock circuitry, or perhaps it
does have it but it's broken?

73 DE WA8MCQ