http://www.theoben.co.uk/customer/pages.php?pageid=3

Sirocco Countryman first appeared in March '86
Countryman Carbine '88

Triggers were based on the Webley Omega unit. The hw35 barrel and BSA cocking link suggestions are, at least dubious, the meteor trigger mech is, if Theoben are to be believed, just wrong.

I'd suggest that you'll get some real detail if you contact Ben Taylor (bengarzy on here). If you have the serial number, usually scribed into the cocking linkage, I might be able to date it for you.

HTH.