I had a letter too and asked a question and was told that the old letter does not apply anymore.
So, you can keep using the old letter since they have not officially told you that it no longer applies and they should have a record that they sent it to you, but their recent letter could be viewed as a change and they might still hold you accountable for the sales tax money.
Personally, I would collect the sales tax since it is not worth the risk. It would be nice to get someone to go after the BOE for this change of view.
FFL Transfer/Special Order Dealer since 1993.
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Never try to teach a pig to sing. You waste your time and you annoy the pig. - Robert A. Heinlein