Hi,
Not sure if you are aware, but you can still obtain copies of Chris Thrale's excellent Webley Air Rifles 1925-2005 which includes a very useful chapter on the Mk2 Service.
He states that the 3rd Series commenced around Ser No 2001 and definitively dates Ser No S3000 to the end of 1934 , so that would probably mean your example was made in 1935 or early 1936.
Out of interest the Ser Nos associated with the different Series are as follows:
Series 1 S1 - S1000
Series 2 S1001 - S2000
Series 3 S2001 - S11000
Series 4 S11001 - S14500
Series 5 S14501 - S15000
Series 6 S15001 - S16300
Hope this helps.
Regards
Brian