Hi Richard,
Sorry I don't know much about where 3&4MLRU operated by my guess would be as follows:
Formed sometime between December 1943 (from 1MLRU) and May 1944, probably based at Portsmouth.
Would have landed D+1 or 2.
Here is the only official mention I have ever found about MLRUs (it refers to ONLY 3&4).
" XVI. Recovery and Repair.
1. Recovery, maintenance and repair in the assault area were under the general control of senior engineer officers on FOBAA (Flag Officer, British Assault Area) Staff. These included base engineer, electrical and construction officers, principal salvage officer, and fuelling officer.
2. Off shore repairs were handled by large repair parties in LSE, Depot Ships and floating docks.
3. In shore repair and recovery was organized on a divisional front basis under the NOIC. His staff included Reserve Assault Group Squadron Engineer Officers and Engineer Officers of the MLRU."
see:
http://www.ibiblio.org/hyperwar/USN/...mNavEu-10.html
Initial port targets would have been Port-en-Bessin and Oiustreham as well as beach support in the British area.
They would have then leap-frogged ports up the coast through France, Belgium, Holland and possibly to Germany.
The Units would have been disbanded as soon as there were no more ports to assault.
Your compressor truck was used to support divers, air tools, salvage pumps and to provide compressed air to refloat vessels and would have ended up being used by port parties along the coast until surplus to requirements.
I have some other photos of my Grandfathers RN vehicles and will post them shortly with some comments on the photos in this thread.
Regard Stephen