Is this a date mark?

[planeguy2]

I recently found this milk bottle on an old RR grade. The mark on the heel indicates that it was made from 1923 to the 50's, so 1927 is possible. The mark I am referring to is simply a 27. Is this a date mark like you would find on most milks or just a manufacturing mark?

 

[CanadianBottles]

I think that is indeed a date mark.

 

[ROBBYBOBBY64]

I think that is indeed a date mark.

Thats not a mold number. How did Thatcher code their bottles? Just asking because I'm not sure. I have mTc with a 29 on the bottom. Is that 1929?
ROBBYBOBBY64.