what the HAY! sunroof?

AVISPA....
Just found the info that you need.

M51 was the code for sunroofs on B-Bodies in 71.

You are correct---V3X is the correct code for an A-Body folding canvas sunroof in black.

Courtesy of Hamtramck Historicals.....

http://www.hamtramck-historical.com...alesmansPocketGuides/1971/71_Dart_Demon_4.jpg

I hope this answers your question.