I have a 77 Series 1, and a 27/FM (military version of the 27).
In the 77 ‘s favor are the rubber holster, which incorporates a tilt stand, belt hook, and probes can be stored on the holster rear. It is lighter and smaller, even with holster. It also has annunciators for VDC and VAC, DC or AC annubiators for current. It is .5% accuracy rated on DC (don’t recall the count uncertainty), and is an averaging meter.
The 27/FM is much bugger and heavier, and is has O ring seals to keep out moisture. There is no holster, so no belt hook, or is there probe storage. The tilt stand is built into the case., and is extremely rigid. Display is slightly bigger than the 77's, and has a slightly tan coloured rear reflectior. There are no annunciators for voltage or current.The 27/FM has uA ranges for both AC and DC, and mV AC range. There are MAX/MIN and REL functions.The 27/FM is a true RMS meter and is rated at .1% accuracy on the DCV ranges +/- 1 count.
Both meters lack a back light, both have nS ranges (manual), both have real touch hold functions (not just display freeze), and both are 3200 count meters. Both are perfectly safe meters (note that the 77 Series I has no fuse on the 10A range). I do like the rotary switch better on the 27/FM; it is the most logical layout I have ever seen. OFF positions at 12 and 6. DC voltage functions on the left, AC functions on the right. To either side of the bottom OFF positions are the Diode/Continuity on the left, and the Ohms to the right.
I’d love to see Joe Smith do his testing on both meters; it would be both a look into the past in terms of build quality, and allow us to see hiw stuff has progressed (or digressed) as time has gone on.
Both are good meters. The 27/FM seems to be all over eBay; the military must be getting rid of a lot of them. There are many 77s available, too, but a lot are Series II, Series III, and Series IV, and they cost more than a 27/FM (Series III are true RMS). If you can live with the increased size and weight, or need a very rugged meter, look for a 27 or 27/FM. Otherwise, a 77 (Series II or later) is fine.