What you posted is absolutly correct, but just because somone can buy a NFA firearm does not mean they are not subject to paying the transfer tax.
If the Stemple was a C&R qualified gun which it is not, then an out of state C&R collector could in fact but the gun across state lines on one form 4 tax paid.
Since you are the seller, any FFL-01 dealer across state lines or in state could buy your gun with one tax on form 4. Any FFL dealer with SOT would buy the gun tax paid on form 4. The FFL each would pay a second form 4 tax if the final buyer was not a SOT dealer.
You might do this as a gun collector owner of NFA but I do it for a living. In this case I am right no matter how many times you point out I am wrong.
I suggest you contact the dealer you bought the Stemple from, ask them if they can buy it back from your tax free. Or contact any NFA dealer in any state and see if they can buy your gun tax free. They can not.