The first letter generally denotes the type of instrument:
RTx - real time (scopes), but note new MXO series of scopes
Fxx - frequency (spec ans)
Sxx - signal generator
Zxx - network analyzer (Z -> complex impedance)
Exx - EMC receivers
Cxx - communications testers
Nxx - power supplies (but note, NRPxx is RF power sensors). I'm told "N" stands for "Netzgerät"
Numbers sometimes (but not always) indicate maximum frequency (FSW85 - 85 GHz).
"Similar" classes and generations of instruments may have similar names, e.g. the older SMU and FSU versus the newer SMW and FSW. Many handhelds end in "H" - FSH, FPH, ZNH, ZPH, RTH, etc. But beyond that there's no universal naming scheme that I'm aware of.
The classic R&S instruments had a quite stringent naming scheme, often with an obvious German origin. Today this has become somewhat blurry. But still, all receivers (not just EMC receivers) start with an E, for "Empfänger". For example
EB200,
ESMD,
EK07,
EK070, etc.
Signal generating instruments start start with an S, and lab RF signal generators still are SMx, for "Mess-Sender", others had a different second letter (e.g. the
SNF video level generator, or the
SPD pulse generator,
SUB audio generator, etc.).
The classic R&S power supplies were all NGxx, for "Netzgerät", whereas power and SWR meters were Nxx or Uxx, when considered a voltage indicating instrument. This went as far as that the URV5 was also available as NRV, the only difference being that the NRV would start up in power indicating mode, where the URV5 would start up indicating voltages. Audio analyzers are UPx. This was of course spoiled by the addition of the former Hameg products (power supplies HMPxxx).
Frequency domain instruments (in a broad sense) are Fxx, e.g. spectrum analyzers, modulation meters (
FAM, FMAB,
FAB, etc.), but also counters (
FEI,
FET, etc.).
The classic attenuators were Dxxx, for "Dämpfungsglied".
Frequency sweepers were SWxx, the most well known are the classic SWOB series:
SWOB,
SWOB1,
SWOB3,
SWOB5.
Communication radios are Xnnn, but some old frequency standards also had designations starting with X. I don't know about broadcast transmitters and antennas, but they sometimes also have talking designations.
Instruments that were manufactured to the specifications of the German armed forces sometimes had an Y in the designation, for example URY, which is an URV5, or the radio tester CMS42y, which is a version of the CMS54. This is probably due to the fact that the license plates of vehicles of the German Federal Armed Forces begin with a Y.
Sometimes there was a little humor in the designations, for example the noise generator
SUF and
SUF2 ("Rauschgenerator SUF"). Difficult to get that across in English.

See
here for an incomplete list of old R&S gear and datasheets.
Edit: The classic R&S scopes are Oxxx, e.g.
OBF,
OKF,
OMF, or special purpose gear like
ODFA.