I also have someone in the institution with the letters CMMG and suspected it was Netherne Asylum. I had the same problem locating the first page, so I tried a search with surname A* and kept trying till I landed in CMMG.
CMMG turns out to be Cane Hill Hospital - another asylum just a couple of miles north from Netherne. It evidently had a capacity of 2,400 patients. It looks like the 1939 register submissions might have been done in blocks of 300 individuals, and males would have been in separate blocks from females.
The first person on the CMMG page was Henry George Abbott born 1868.
I haven't found any proper definition of the codes so this is a working theory.
