Hello again Pete.
Yes there are lots of abbreviations but most don't seem to relate to units or any other kind of identifiable group.
The last diary entry is Friday 23 Feb 1945, so unfortunately doesn't tie in with your Irrawaddy idea. On this day they arrived in Monywa at 1430 and were "met by Capt Sangdon". This entry also mentions that he watched ... "Libs and Mitchells going over to bomb Mandalay et al."
I'm not sure if this will help but I know one of those in charge (I don't know his title) was a chap called Mingaye. An entry covering the days 12 to 17 Feb 1945 mentions that Mingaye tells them "he's been posted to 243 W/T". A couple of days earlier Mingaye flew to Monywa, and Hoekstra (this is my best translation of his handwriting) was their new OC.
Obviously I've googled these names, found this on National Archives Documents Online, and wonder if this is the same chap:
Name Mingaye, Harold William Frank
Rank: Temporary Captain
Service No: 268268
Regiment: Royal Corps of Signals
Theatre of Combat or Operation: Burma
Award: Member of the British Empire
Date of Announcement in London Gazette: 06 June 1946
It's an interesting puzzle to try to solve and I've learned a lot along the way.