1937 transfer between regiments

The man that I am researching joined the Royal Artillery sometime in 1933. His WW2 POW records shows that on the 10 January 1837 he was transferred into the 2nd Battalion of the Gordon Highlanders under Para 204(15) TAR.
Can anyone tell me what Para 204(15) TAR stands for?

 

Not sure if this thread is of any help?

TA Regulations 1929 - paras 207 and 204

 

Thank you Martin, this answers my question.

Cheers,
Allan