Agreed that the letter goes with the gun.
Having said that, if you - for whatever reason - need to reference a gun that you "used to have", just make a copy of the letter for your files and send the original with the gun.
I've referred to info on past owned guns that I've lettered, it comes in handy on occasion.
On the other hand, I have received letters on guns I've purchased and have really appreciated having the documentation, and it reminds me who I purchased the gun from (my memory being what it is, this really helps...
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