Hi H2C
That number was used by a number of KLR battalions, and none of them appear to have entered France in May 1915. But the post card is the key. It gives his battalion as the 4th. They were pre-war Special Reservists (ie part-timers) who became full-time at the start of WW1. They went to France in March 1915, so he was a later replacement.
I can tell you that 10387 was issued in the 3rd/4th Battalions in August 1913, so 10912 have been issued near the start of WW1. It may even have been issued before WW1 started.
Their uniform was very similar to most of the other battalions. Only the shoulder titles were different. So search for Liverpool Regiment or KLR and you will find something. You should ignore the Rifles (6th Battalion), Liverpool Irish (8th), Liverpool Scottish (10th) and the Liverpool Pals (17th to 20th Battalions).
Ken