This is from my Jazz mail list:

"The tune was first recorded in 1941, but had been written some time 
before.
Perhaps 1938 is the date of composition?  Strayhorn didn't think much 
of it
and it was unlikely to see the light of day until Mercer Ellington 
found it
and championed it."

The IND and BMT used letters from 1932. The 'G' originally was 'GG" 
like all other locals.
When I briefly lived in Queens and attend Pratt University it was still 
named the 'GG.'
I do not remember when the locals lost their double letters.

The 'GG' as far as I can research online was always a Brooklyn-Queens 
line.