The Diana 5 (standard) and 6 (recoilless) pistols were first introduced in the late 1950's. As far as I know the model 6 was the first truly recoil-free spring-piston handgun ever made--IMHO it's a significant milestone in airgun history.
Initially both guns had a one-piece grip/frame unit made of wood. Shortly this was changed to molded plastic. In the mid-70's, the guns were revised with a new cast metal frame having separately detachable grip panels, designated models 5G and 6G. These are still around. As you can see from the month and year stamped on the left rear receiver, your Beeman 800 is an early-build model 6G.
Later Diana developed the advanced model 10 match pistol, combining the model 6 recoilless powerplant with a highly-adjustable trigger, anatomical grips, and an open front sight (the 6G's hooded sight is not "legal" in most formal 10-meter shooting). A "legal" version of the 6G with the model 10 barrel was sold as the model 6M. Many later 6G's and 6M's were sold with adjustable wood grips.
Beeman sold all four guns:
Beeman 700 = model 5G
Beeman 800 = model 6G
Beeman 850 = model 6M
Beeman 900 = model 10
As Dave mentioned, the Beeman / Diana alliance turned out to be short and unhappy! So your 800 is a fairly rare beast. Diana used many different trademarks in different markets, or for different distributors; you will also see the pistols under the names Geco, Gecado, Hy-Score, and Original (and maybe more!).
This pic shows the first-version model 5. You can see both the similarities and evolutionary changes when comparing your 6G/800.
