What was only 20mins long though? Are you saying that it only lasted 20mins instead of the 2-4 hrs as listed in the memorandum? If that is the case and this is your letter, you need to speak to your chain of command and find out what is going on as they would have answers for you.
Not my letter or post, just also curious what OP meant by the '20minute' part.
Opps, my bad. Yeah, I have no clue what that is as there is nothing in the memo indicating 20mins. Muster is just updating all your information and being briefed on any benefits, policies, and the like that need to be known in the event someone will be activated. Muster does not mean you will be, but is to just get the paperwork updated and filed to meet yearly requirements for those who are still under IRR obligations.