81st HAG War Diary - Meaning of MX and PX ?

[Mike999]

Hello all. To give some context on 25th July 1917 81st HAG were supporting the Passchedale operation with artillery from their Siege Batteries who I believe at that time operated 6" howitzers. The guns had just moved to a new forward position and the diary entry is below. It states they had "put 920 MX into 202SB and 600 PX into 210 SB". SB is short for Siege Battery.

My question is what are MX and PX? I assume they are short for some ammunition type but don't know. Can anyone throw any light please? TY