I am not an expert. It appears to the the correct scope for a Lee Enfield #4T sniper rifle.
There are rifles around with the original rings but missing the correct scope, so these scopes are very desirable.
I would guess (but don't know) that this scope is worth around $1,000. One way to find out real value is to use the ebay history function to find out what the scopes actually sell for. (And don't sell to the first guy who offers $1,000 either. It could be worth more. A lot will depend on the maker and condition.)
An old scope like this probably needs a service to free up the knobs. Don't force anything.