[time-nuts] HP5065A Step Recovery Diode, what part#?

[Chicken Time]

Greetings,
Long, long time reader/fan, first time writer.   I have a HP5065A that I've been messing with that has a problem which I think I've narrowed down to the SRD.  It is functional but seems to have an unusual behaviour, including visible amplitude variability and flickering of of the 2nd harmonic.

I've replaced everything with guts of a 2nd unit but nada.  The last step in my testing was to replace the SRD, but the only ones I could get from easy sources were SMT ones. However, I was able to hack one in!  This did work and the 2nd harmonic was stable!   BUT of poorer quality Allen Dev with an original Axial HP step diode, of which I am not entirely surprised.

The HP SRD has the text H391, 391, and P391 written on it.    Nothing in the Manual or Google searches gives me any indication of what it is, including combination such as taking the number as a resistor (390).  None of HP's original list of SRDs is even close either.

Does anyone here know what IS this SRD?
Thanks much,
Dan

-Chickens are more obsessed with time than Humans. Proof: Clock Clock clock