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The difference is the lifetime map subscription (LM) which is included with the Garmin Nuvi 1390LMT.
The unit will not have any designation letting you know it is an LMT. If it is either a 1390T or 1390LMT it will be written only Nuvi 1390 or 1390T on the bottom. Just try to register the unit with Garmin and activate your lifetime mapping subscription.

And considering that the lifetime period has long since expired, it doesn't change anything for you
Lifetime is not the period of your life, but the period of the expected lifetime of the device.

It seems to me that you are hooked on used Garmin devices.
Make an effort, try more, much more. Start reading a lot.