Dating Webley MK3 by Serial Number  

In the Colletor's guide to Air rifles by Dennis E. Hiler 4th edition, the Mark III series II started in 1957 up to 1961. Series 4 from 1961 to 1964. All serial numbers are numbers.

Mark III late model 1964 to 1975. The loading tap of this series is straight, whereby the older models were very nicely rounded  machined steel. The serial numbers of these guns started with the letters A and B. So, I will say that on your gun the loading tap lever is an ugly straight line machine cut. (I have one of those too. No insult meant). It seems they had 4 digits after the letter. The B serial numbers seem to be for guns fitted with anschutz target sights. Could also be a Supertarget. On the Supertarget there is small round cap at the bottom of the stock, under the trigger. It should contain some front sight inserts. Hope it helps!