Beggining of 1954 the IRT and Maihak made several units(about 50 or so), that were just labeled V76, but had all the 80 low and 3kHz high cut.
The historic TAB company started in 1956 with the production of the V76 version as mentioned above, adding the V76 with 120Hz cut in 1960, since than there is the 80 or 120 extension on the designation number.
The 80 unit was aimed for the radio market, the 120 for TV studios.
Both units are the same with the difference in the filter choke that set the cut off frequency.
Also in 1960 the "S" version was introduced, "S" stand original for "Schallplatte" or for Vinyl Disk, indicating "Studio use" and in 1965 official changed to "Studio".
The "S" version is quite different, multiple switchable low cut, and also multiple switchable high cut. Filter inductor are made from NI/FE alloy for better sound performance than the standard ferrite cores in the 80/120 units.
The input x-former, even barring the same part number, it is similar to the standard version, but hooked up in a different way, also a bit differently wound. There are also a few minor changes in layout, wiring, etc.
The "M" version was released as a reference amplifier or better measuring amp indicated by the "M" the german word is "Messverstaerker". Technical speaking and entire different amplifier, output and tubes are the same, input and front amp are different as can be. Input x-former is 10kOhm impedance, and max. gain is 60 dB. About 600 units were made.
In 1968 the "MT" was released the "M" version as technology/laboratory amp, with a frequency response 5 to 50k Hz. a bit different than the M version in circuit tree, and 66dB of gain. Less than 100 were made...
Nearly at the end the "FR" version was designed for better console layout only used in 24 plus channel consoles with remote controllable gain. The standard 80/120 chassis without the filters...
There are a few custom version made over the years, and it is worth mention that there is an EMI made V76S with a few different options, x-former are the same than the TAB made "S" version...
Hope that helps....