I have a 1983 HW77, I bought it off sales on here some years ago!, I was told that they didn't actually start importing them here until 1984 so how this got here before i've no idea!!
The 100375 serial puts it at 1983, This was confirmed by Weihrauch too as '83
It's been well looked after too
HW77
EDIT- Ooops!! Missed the .177 bit! Mine's a .22 cal
John