Apple sold an upgrade from the 512k to a Plus which basically involved swapping out the motherboard, floppy drive and rear case (since the port layout was different) with new components. I'm guessing this is what you've got.

As for the 2MB upgrade... since the Plus motherboard takes standard 30-pin SIMMs, 2MB is quite common. You could even upgrade it to 4MB for next to nothing by finding 4x 1MB SIMMs.