I had some spare time over the weekend so went through all my United memorabilia from the 1940's to early 1970's......
Although always fascinating in itself, I was looking for 'Man Utd' references.
This includes, programmes, newspapers, magazines, brochures and even cloth and metal badges.
In the 1940's and early 1950's the term 'The United' was used by the likes of Alf Clarke, Tom Jackson, Butterworth, and others. This was also very common pre-war.
After Munich, David Meek, Tom Tyrell amongst others used the terms 'United' or 'Manchester United'.
In the late sixties and early seventies a lot of references used the abbreviation "Man Utd' or 'Man United'.
I couldn't fine any references to 'Man Utd'.
That is not to say that it wasn't used at the time, but I certainly can't find any record of that term printed in any way.
This doesn't support either of BR's or FS's arguments necessarily but at least it is some accurate data.