While I'm not an advance collector of airguns like some of the posters on this board, I did compete with a FWB 300S and a FWB 600 for a number of yrs, so I have some knowledge of FWB airguns and have seen more then a few of them, AFAIK there are 5 versions of FWB 300.
1. 300 the first version.
2. 300S the basic version, with fixed barrel weight.
3. 300S Junior, without the barrel weight.
4. 300S Universal with adjustable cheekpiece.
5. 300S "L" version, same stock configuration as the UIT standard rifle.
Then there is of course the running target version (laufende scheibe) 300S LS. Mechanically they're pretty much the same, with minor differences in trigger adjustment; sights; stock configuration...etc.
I have never heard of a FWB 301, that of course doesn't mean it don't exist, lacking a photo, can you describe what is the distinctive features of the FWB 301? sure would like to know.
Your notes here sum up the FWB 300 very well. There were some minor variations in stock designs between early and later models, but the designation remained the same.
To enhance your info regarding the vintage FWB springers,
FWB's very first recoilless springer was the model 150, introduced around 1961.
The model 110, introduced in 1962, was identical to the 150, but without the recoil compensating mechanism - an "economy" model, so to speak. Only about 200 of these were made.
The FWB model 200 was identical to the 300, but again, without the recoil compensating mechanism - likewise, an "economy" model. Another rare item to find.
The 300 and subsequent 300S, (and its variants) were the very last springer rifles produced by FWB.
FWB also produced a few models of sporting springer rifles - the model 121, 124, 125, and 127. The 125 is a particularly rare beast, in .20 cal, which was a special order by Robert Beeman, and of very limited quantity.
Only one unusual "prototype" FWB 300S based gun has surfaced to date, but the markings are distinctly those of an FWB 300S. I have pics of this particular gun, as well as the comments of the individual that owned it at the time. It has since changed hands.
I regularly scan the German eGun auction site, and a few other European airgun websites, and in the past 10 years, have not seen a single reference to either a FWB 301 or 302. According to my sources, the FWB design team was more or less forced to drop all spring rifle development with the introduction of Walther's LGR - FWB had to play "catch up" with the Walther technology to remain competitive.
Could be that the 301 that dizzy alludes to was a one-off toolroom prototype that was just arbitrarily dubbed that by the people working on it, but never officially recognized as such by the company. IMO, I really can't see FWB going to the expense of setting up a roll stamp for a one-off gun, but then again, stranger things have happened.