McGonagall first subject

[kmcbears1]

No: HPFGUIDX 114521


In OotP, McGonagall tells Umbridge that she had been teaching for 39 
years this December. Using the dates from the HP-Lexicon, McGonagall 
began teaching in 1956.  Dumbledore did not become Headmaster until 
1970.  Dumbledore was the Transfiguration teacher when Tom Riddle was 
at Hogwarts in 1942. 

What subject did McGonagall teach when she started at Hogwarts 
December 1956?

If she started as the Transfiguration teacher, did Dumbledore teach 
anther subject like DADA or Potions?

- kmc