Family belief is that Thomas had been born under a different name and have his birth year as December 17 1912.
The Manchester birthplace agrees with what the family do know about Thomas.
As adoption didn't become official until 1926 I expect this may never be discovered.
A long shot if Thomas Mellors had a completely different name at birth there are these births entries registered in
Manchester Dec Qtr. 1912 on FreeBMD with the
same mother's nameHOLLAND Charles A Holland Manchester 8d 429
KELLY John Kelly Manchester 8d 402
MONKS William Monks Manchester 8d 324
PRINCE George Prince Manchester 8d see D/15
Dec 1915 PRINCE George W. Prince Manchester 8d 346
The above transcription represents what is in the GRO index but it is referred to by the following entry Births 1912 Dec: PRINCE, George, Prince, Manchester, 8d, see D/15
which implies that this is a late entry, that is an entry that has been registered in a quarter later than the event
and if the birth was registered in the Mar Qtr. 1913 there are these entries
KNOWLANDS William H Knowlands Manchester 8d 400
MELLOR Stanley Mellor Manchester 8d 403
SMITH Harold Smith Manchester 8d 433