Appears everything under the hood is factory original and not messed with much. Which is impressive. The compressor is a 198, check. On top of the Evaporator housing, you can see what's left of the replicated ECM decal (what the techs could scan instead of having to pull out the ECM under the dash), check. The air cleaner code decal, check, even the A.I.R. system appears in tact. The compressor hose from the dryer can is likely original since there's a black nut on the hose end.
My experience with G-bodies tells me that you either 1) have a locked up compressor (this would be really bad) or 2) like you surmised, someone thought they could save 2 HP by taking off the belt on a system that has been depleted but never recharged or leak found, etc. (which isn't great but nothing you can't fix).
I agree with airboatgreg in that you need to see if your compressor turns freely. If it does, then check the entire system over to check for obvious leaks before just buying a belt and throwing it on there. If you see lots of leak areas, they'll likely have mineral oil around the leak. This is what keeps the compressor lubed. Without it in the system, and you put a belt on there and bypass the low pressure cut out switch to kick it in, you may torch the compressor and REALLY have a bad day.
If it's locked, then you know you'll either need to get your compressor totally rebuilt, or find a replacement. Thus, if it's locked, then that means you 99.98% likely to have metal shavings floating through the system. You can likely flush out the hoses, but not the dryer. The orifice in the small line going to the evaporator core is likely clogged with crap. Either way you'll need a new one of those. You'll likely need at minimum all the O-rings for the system. Your condenser coils in front of the radiator may be cheesed as well if it's locked. Professionals may be able to flush it, but I bet it's original which means the S type tubes vs. the cross flow they make now which would work better if you were converting to 134A. The supply pipe from the condenser to the evap/orifice tube can be flushed. The evaporator itself can be flushed as well. Then you need to evacuate the entire system in a deep vacuum, and you must ensure you get out every single bit of mineral oil because it does not mix well with PAG oil in a 134 system. Unless you use ester oil. I have heard, but haven't tried, ester oil in a conversion, but it's supposed to be compatible with mineral oil and R12 and 134A systesm. I was told that the aftermarket compressors you get at the auto parts stores come with ester oil for this reason. And the hoses may hold up, but likely the rubber hoses for R12 will bleed through using 134A. So to do it right you should replace the hoses with 134A suitable hoses.
Also for conversions, get the heaviest duty compressor you can. The little compressors that came on G-bodies came in two styles that I'm aware of. The light duty and regular duty (like your factory one). 134A runs at higher pressures and MAY be rougher on the light duty ones. Just something to think about.
If you haven't had much experience with it AND you don't know what you're starting with AND you're converting to 134A, I would highly suggest talking it over with professionals or someone who's had a boatload of experience doing this prior to doing anything.
And remember, even thought there's a lot of us who has jacked around with A/C systems on these cars, we're not there. It'd be after the fact if something goes wrong and if we forget to tell you that one little detail that makes or breaks the system, then oops. Validate everything anyone tells you on here. There are books written about this stuff, so it's hard to diagnose and get it fixed with just a few paragraphs of info.
Not trying to dissuade you from doing this, just trying to pass on some advice about how not to destroy a car A/C system in 30 seconds or less. (Ask me how I know).
