As I am sure you are already aware, Purdey did use an A prefix when numbering rifles retailed by them but not of their make.
At one time I owned a .22 Savage High Power 1899 take down which they had sold to Lord Tredegar. It had their number (think it was A1281?) engraved on the guard bow.
In Christopher Austyn “ Classic Sporting Rifles” at page 77 he shows a Purdey cased and retailed 8x56 Model 1908 MS, Steyr number 3005, Purdey number A1228 retailed by Purdey in 1912.
I have much appreciated the information you have posted on loading for the 9.5mm. My 1910 Stutzen is proofed as .375” and was retailed by Watson Brothers in 1914.