I could be wrong, but if you purchased the handgun in another state prior to moving to Maryland and didn't register it when you moved here, then you may not be able to use it as an exemption to training. If it is possible to do, you are letting the state know about a handgun that they had no previous knowledge of.
The law simply states if you owned a regulated firearm before Oct. 1, you are training exempt. It makes no mention of where you purchased the firearm or whether it was ever registered in MD.
If it's not specified, it's not in the law.
IANAL. Nate is.