The number is not a serial number but a batch number. The Junior MKII was manufactured between 1973 and 1976 (most in 1974) total produced 31,750.
This info from the Bruce book.
As to value, something that has long been debated here, it is a Webley and this forum seems to think the breed sacrosanct (ignoring other rare and foreign breeds) so if you sold here . . ..
Probably, if mint, £70????? (boxed? Mine is and unwrapped.)
At the end of the day value is in they eye of the collector, how badly do you want it?????