what size fittings

No it is not. Here in the U.S. it is "called out" as 5/16 SAE flare, which refers to the standardized fittings for 5/16" outside diameter steel tube. There are adapters or "half unions" which convert from that fitting to whatever the thread is directly into the trans case, I forget, either 1/8" or 1/4" US pipe thread. Again, this is a "call out" correct designator, and NOT the actual size of the fitting

The adapters into the trans case should look something like this, except steel, not brass

http://www.suremarineservice.com/images/products/detail/FIT332.1.jpg

5/16 SAE flare is listed as 1/2" x 20 thread, and again, these are 45 degree flare, not the 37 1/2 degree flare which JIC/ AN fittings use.

By part number these are often referred to by the "dash" number which is nothing more than 1/16" increments That is, a fitting series for 5/16" OD tube is the basic part number followed by -5. This is where you hear people say stuff like "using no 8 fittings" meaning -8 or 1/2" nominal series. One inch fittings are -16 for 16/16"