Let the speculation begin .....
Stan Lee, Steve Ditko and Marie Severin have all been removed from the donor list by Sarah Duke, the curator.
As someone else suggested, perhaps a proxy.
Stan was a showman. If he owned it, he would have displayed it long ago. So none of his proxies.
Ditko had abandoned his rights to the art and essentially gifted everything to Marie Severin. So none of his proxies.
The general consensus among collectors was Marie Severin being the donor. Marie never married, nor did she have any children. Perhaps a niece, nephew or family attorney.
Otherwise, from a pure speculative guess ...... What If ...... the same person in possession of the complete ASM # 1 art also managed to get a hold of the AF # 15 art? What if that individual made the anonymous donation?
Somebody please ask the individual or Sarah Duke. Lol.
Again, pure speculation on my part.
Sorry about font issues.