For these the date could be stamped on the left side of the barrel breech or maybe the cylinder, can't be sure also could be stamped in the butt end of the stock, 4 digits month and year, although often worn away as they get stood on their ends on rough surfaces quite often, I'd expect early 60's and think they were mid 50' to mid 60's. On my one the breech has a serial and the butt has the date (1960).
As they are small junior sized the following is much lesser than full size types of the period.
In that condition value will be low. Send it to be restored & you'll spend far far more than the end value, do it yourself and the time involved will not match the return plus you'll probably have to purchase seals & spring.
I have one as the second owner I got 30 years ago in quite nice condition for £20 at the time but if I took it to a gun shop sadly I reckon I'd be offered well below £100 if they even made an offer because I did once touch on this with one buyer/seller. Shame to think this and of course just my thoughts, ideal to practice restoring on though.