I had a bat made by MG2 a couple of years ago and requested a McCullum profile. They said they had repaired a few of his bats but didn't make them. I would assume, as in NZ the only other top brand is Laver, for McCullum never to have had a bat made by MG2; he has probably been using the same company for years (I.e. Laver). I notice the SH was printed on the bat handle, not sure which companies do that but it should narrow down the bat maker as I know Puma, MG2 and Ton (I think) don't print/stamp.
In terms of the bat it lasted two years and was a belter. It weighed 2'9 and had an excellent pick up. Huge middle/profile. Minimum concaving. Nothing particularly special about the bat, as in it was a middle-high sweet spot and wasn't extravagant but it did perform and it was pretty forgiving.