HP 400E AC Voltmeter - Clone???

[wn1fju]

Picked up this AC voltmeter for $20 USD.  There are no identifying markings except on the bottom (see photo) where it looks like a tag (AUL Instruments ME-30F/U) has been cut out from somewhere and taped to the case.

The front and back look similar to an HP 400E meter, so I compared the circuit board to the parts layout shown in the HP 400E manual.  It is virtually identical down to the component numbering.  In fact, the only obvious differences between this meter and the HP version seem to be:

1) an additional adjustment pot for the P/S voltage
2) the P/S outputs 22 VDC, not the 26 VDC in the HP 400E.
3) an internal bias level is adjusted to 8 VDC, not the 10 VDC in the HP 400E
4) an additional 3 MHz adjustment pot
5) obviously different meter movement and case design, no ground lug on the back panel.

The meter worked fine after I replaced the old-style HP AC jack that was all burnt up and missing the center pin with the more modern type.

But, my question is:  did HP license this design?  Was this an unapproved "copy" of an HP 400E?  The ME-30F/U is a USA military ID number, but why wouldn't the military simply buy the HP 400E from HP? 

[xrunner]

Probably some military requirements they wanted in the meter from hp, as you noted there are differences. Why they wanted them  ... who knows now?