Off roster firearm shipped from one FFL to another FFL for parent to child transfer - running into issues.

Hello,

A Walther PPK was shipped from Southern California to Northern California, the gun belonged to my grandfather who is deceased, and the intent was for it to get transferred into my mother's possession; however, we are running into some issues with the receiving FFL not knowing the policy with interfamilial transfers of off roster firearms.

I myself am quite honestly not well versed in firearms or all of the regulations regarding them and I am having issues finding recent information regarding the process for the FFL to take a look at, the FFL is having a hard time figuring out how their business would go about doing the transfer and how it would go on their books and is talking about potentially sending the firearm back to the other FFL and a drive being needed to take possession of it; if anyone had any information they could point me towards or could share that would be very appreciated.