Seems like there was one subsequent "portable" FI meter - Singer? before the Nems-Clarke (also RCA) meters became the standard. Mine works and is in very nice shape, but is missing the battery plug. I think the plug was made by Cinch.
These meters can still work pretty well, but were supplanted by the 70's vintage FIM-41 / 21 series still in use.
I've never figured out how the calibration oscillator maintains amplitude stability over the very wide temperature range it encounters - there is a 1n60 diode from the grid to ground, and I know those germanium diodes' forward threshold moved a lot with temp.
K4KYV gave me a connector for it, but turned out to have the wrong number of pins
That plug you pictured has six pins but the center one doesn't connect to anything. The matching jack has six holes but the center hole has no connector. Let me know if that is correct on your meter.
I just rechecked and I think I also have the ones with all six active pins. They made those connectors with 3,4,5,and 6 pins. both male a female. Some are chassis mount, some have a metal cover. The inserts can be changed around as they are held by a small screw.
Let me know if the meter uses all six pins or just five, if you can tell. I see I only have the chassis mount six pin type. These can not be used with the metal cover.
As I remember I think these type connectors where used on battery packs.
I'm assuming you need a matching jack (to your pictured plug) that will be connected on a cable running from a battery, or if not then what??