Several years ago, the seller's form 4 had the 21AC listed as a 28AC.
I called ATF, and was told to put the incorrect information on my form 4 application, "...otherwise, it will just complicate things and add six more months to the transfer".
I repeated this back to him, to make certain of what he was saying. Yes, that was what he said.
I replied, "But, then I'll have a new form 4 with wrong information". Reply: "Well, you can get it changed after you have the approved form".
I asked, "Will they send me a new form 4 with the correct information? Reply, "No, they won't do that. But the registry will correct their information".
So, that's how I did it.
After the transfer approval (over a year later) I sent a letter explaining all this, and requesting the information update.
Around another year later, I received a one page letter, stating only:
(Quote) "The requested change has been made to the registry".
Nothing else. No serial number. Nothing to even indicate which item or change was involved.