I have seen quite a few Center at the Kempton Fair. They seem to show up quite regularly. I think a Relum air pistol shared the same underlever cocking with the swing out gate for loading. Only designed for very short pellets.
Always remember seeing boxed examples in the local gunshop window as a teenager. Never got one - always had older Webleys instead.
Lakey
If you are interested in taking a punt on the Chinese version, I have just noticed that Okehampton Guns are listing one on their website, priced at £60, reduced from £90.. New but shop-soiled. Down in Devon though, unfortunately.
https://www.okegun.co.uk/product.php...k=:::6445200:0