Michelle.
A soldier keeps his Regimental Number no matter where he moves to in the Army so the one issued to him for the Black Watch will have been the same as for his time in Ox & Bucks LI.
6th Battalion Black Watch were at the Fourth Battle of Monte Cassino in May 44 as part of 12 Infantry Brigade who were part of 4 Infantry Division who were themselves part of XIII (British) Corps.
4 Infantry Division had the tricky task of crossing the Gari/Rapido river. They did with 28 Infantry Brigade and 10 Infantry Brigade in the initial river crossing and 12 Infantry Brigade in reserve to exploit the success. It is likely that he was wounded in that exploitation phase as 12 Infantry Brigade pushed towards Rome in full few of the monastery which, at the time, was still full of Germans.
Regards
Frank