Checking for MICs at least 10 numbers either way, I found less than 50% on straight numbers and even less with the WR prefix. All those with plain numbers are Pioneers, all those with WR/ are Sappers.
Checking several surviving service records:
341535, 341539, 341555 and 341561 all mobilised/posted mid-January 1918
WR/341535 (formerly 338548) and WR/341540 (formerly 338552) ditto.
By the way WR/341540 was also an iron worker.
Phil
