I'm not sure if R-12 is legal any more. The Government have been changing what gas is legal in an effort to reduce ozone depletion even though the gas is heavier than air...
You can fill it from the rear, yes. Best bet is to talk to a refrigeration guy who may still have some gas, strictly under the counter of course.
It may be that you will have to change the gas to an alternative - this can be done but only by people that know which gas to change it too as you cannot change to R-134a as it will leak due to its smaller molecular size.
There is an alternative gas (according to some websites) that has added oil of some sort.
Hope this helps a bit.