I don't know how much this is worth to the heirs, but I've explained their options to them - including trying to install a longer barrel to make it restricted and deactivating. I did not get a feeling that the pistol is important to them as a keepsake and given its value...
I'm not the heir and it's not up to me, once the estate is past probate and if no one is interested in taking the trouble to register it, I'd offer them $50 and try to get it registered and then 12.7'd myself just to try out the process - the more I know about it the better I can consult on this subject.
Sorry, drslav, I was not able to get any more pictures before OPP swooped in. It is in their lockers now. Usually we get the prohibs back at some point in time for assessment. If I'm able to get more pictures I will post them.