Chailey Parish magazine first mentions William Mainwood in March 1916, noting that he has attested for service. By October that year, it records that he is serving with the 9th Royal Berkshire Regiment in England. The medal index card that I mention on his page gives the number 27043 and I see from the data that I hold that that number would have been issued to a Berkshire Regiment man in May 1916. That would seem to fit with the information we know for fact: attests in March 1916, is called up shortly afterwards and goes to the Royal Berkshire depot in May 1916 where he gets his number, posted shortly afterwards to the 9th Battalion.
My data on the Ox and Bucks numbering sequences is a little thin but the second number that I mention for William Mainwood - 33201 - would not have been issued until after March 1917.