I have here in an appendix to 21 Army Group RA War Diary 1944 (WO171/155) the following: "EXTRACTS FROM A REPORT BY SI A/TK SCHOOL OF ARTY WRITTEN AFTER A VISIT TO NORMANDY IN JULY 1944" What on earth would "SI" be? I'm starting to think that I should have bought a copy of Royal Artillery Glossary of Abbreviations: Historical and Modern by Philip Jobson which I saw in a shop in the UK during my last trip. Does anyone have it and do you find it helpful? At the time it looked like overkill.
In modern day terminology (cadets etc) SI is usually Sergeant Instructor (i.e. SSI is Staff Sergeant Instructor). However PSI is/was Permanent Staff Instructor in TA units.
British army abbreviations are a minefield, the same abbreviation having different meanings in different branches. In an RA context SI was and still is Senior Instructor. Regarding the book, I can't recommend it highly enough, although I am biased, I wrote it!!!. I am always happy to help with abbreviations, my working document is three times the size of the published book. Phil
PM me an email address and I will send you a copy of the working document (warts and all), but please don't share it, I still hope to publish a second edition one day.