I think we can narrow it down to a 1981 model year? That last picture you posted appears to have an 81 number just below the 915 in the 915.301.301.6R casting number. So if its 1981, then it would probably be a 915.63
Looks like you may have pressure washed it? Make sure to check inside the diff housing that you didn't get any water in there. Old output flange seals can be pretty leaky. So it's not unreasonable to think you could have blown some water past the seals. The same applies to the shift shaft at the nose of the trans because that seal is also notoriously leaky. You also could have blown water past the input shaft seal in the bellhousing. Buyer will also need to polish the tip of the input shaft to let it slip into the flywheel pilot bearing- the rust on there may create some interference.
Another thing worth noting about this trans is that it has the latest side cover. The last iteration of this side cover added more ribbing placed midway in the radial ribs to further reinforce it against flexing.
https://www.wevo.com/Products/TransmissionProducts/WevoTransmissionProducts-External_TransmissionSideCovers.htm
http://forums.pelicanparts.com/porsche-911-used-parts-sale-wanted/857373-915-diff-housing-cover-84-model-year.html
Also not a bad idea to take off the drain and fill plugs just to ensure they come off. They're a tapered thread (like NPT pipe thread, but metric) and it doesn't take much grunt to overtighten them to make it really tough to get them loose. Plus you can see how much sludge the magnetic drain plug collected.
Not a bad idea to take off the tail cover to get a look at the condition of 5th gear and reverse. It's no more difficult to remove than the side cover. Just be aware there's a pin for the reverse light switch that you need to catch when the tail cover is removed. The pin has a narrow end that is inserted into the hole of the switch. The pin is activated by a roll pin sticking out of the 5th-reverse shift rod. Rather crude but reliable.