Then give them the S/N on the Ruger. You'll be good to go.
So this is an exception for new residents after 2013. Great!
It's not really an exception. Bringing a non-prohibited gun into Maryland as you become a resident is fine. The restrictions relate to transfers/purchases, not current ownership.
Do I need the training? The "proof" they are asking for is a firearm's serial number that was owned prior to Oct 1 2013. I have handguns owned from before then but only my Ruger 22/45 has been owned that long. I registered it when I moved here this year.