If you can load a pellet singly, (and do so), if it fits the box, and lifts the 500gm weight, and the grip is legal (rules on palm shelf limits), and a standard grip is legal, then it will pass EC, its legal and you could use it.

That said, as a 10 mt single shot match pistol, its not over competitive compared to a modern PCP, or even a true single shot match pistol of any age, but they are great fun. We had one as a club pistol for five shot, the trigger is a a bit agricultural, and from my recollection of our old club one is it would lift the weight easily, its was quite heavy, if you tried to get the trigger even close to 500 gms, as soon as you touched the trigger it would go fully auto!

Brrrrrrrp! All gone! Great fun but a bit disconcerting for those not aware or expecting it!

Have Fun
Robin