So I re-ran "Fast IMG Unlocker v.1.5 update 12-12-2010" on the same source img file, and I could not reproduce my earlier problem.
Everything looks like it is working just fine.
Something glitched on the first attempt and I cannot tell you why it happened.
It was such a weird failure. The only thing that was missing was the street names in the "Find Address" function.
Streets were there in the browse map, you could route to addresses, POIs were all there, all the City Names were there.
And the problem was not unique to Nuvi. The 76Cx had the same problem with the same img file.
Neither GPS complained that the img file was corrupt or there were any problems with it at all.