The A999 transmission was better than a A904, built with more clutch disks, etc.
It also has the lower First gear IIRC 2.74, over the 2.45 regular 904 First.

They started being made in the mid '70s, according to my old Direct Connection chassis book.

The A904, A998 and A999 were made in the same case, so one would have to check the number on the pan rail to see what it exactly is.

R.