What is this FWB

Hard to tell for sure, as some details like the trigger are obscured, but it looks like a 300S action in an earlier model 300 stock. And note it’s a left-handed stock.

The first FWB recoilless match rifle was the model 150, made 1963 - 68. The 300 was made 1969-72, then the best-known 300S from 72 to the end of production. The 300 had a more “sporter-like” stock than the 300S, and a longer barrel than either the 150 or 300S. The 300 typically had a longer cocking lever with a swing-out latch, instead of the push-down latch on the 300S lever.

This shot will let you see some 300/300S differences. Top to bottom:

“Frankenstein” 300S! A Mini action in a 300SL stock, with aftermarket buttplate.

A 1974-vintage 300S Match.

An early-80’s 300S “Generation 2” Universal.

A 1969 model 300 with beech stock (and its barrel sleeve removed - note the sleeve is straight, no fat stepped end like the 300S).

Agreed 300s in an older 300 stock. Are there any stampings on it? The serial number may also be helpful to get closer to a manufacture age.

I am going through one right now. Unless you know the seals are good I wouldn't shoot it more than a simple function check. You can easily damage internals without proper back pressure.

Man" that's nice stuff" looks like a cross over 150 to the 300?its a keeper"

It was built in 1980 . It's a 300s in a 300 stock . If you wanna dump it give me a shout  

Stock looks like it's a keeper , probably why it's been Frankensteined. 

There are all kinds of mixed bag FWB rifles out there from years of broken stocks along the way , and shooting clubs swapping parts . Including a few " transition" rifles when the 300s first came into production some leftover 300 stocks were used . It's not a guess by the way , got that straight from an Feinwerkbau engineer . 

Most of the half n half rifles were sent to dealers at a discount , or given away as promotion pieces . I had one and OC Bolding has one also . On those the serial number is marked inside the stock below the cocking lever . 

Your gun is a beauty , enjoy 

Willy: your observation is very true! The 150 stock was quite similar to the 300, but the earlier rifle's grips are checkered rather than stippled.

Half breeds are nice. JM Arctic spring. I did not like the Jm buffer being tall. Lathed it down to match the factory and no complaints. Mine is a Daisy branded.

You might want to place your inquiry on the American Vintage Airguns Forum. A lot of folks there are serious collectors. The Forum also has an extensive photo "Gallery" to help identification.

Your rifle appears to have a l/h stock, which might explain why it was switched. As a lefty, I know from experience that these "wrong-handed" stocks are often difficult to find.

Beautiful rifle. Enjoy!