It probably is a custom job, but it does sound like it could be from the mid '70's.
Did the logo look anything like this?
A lot of times, the elements take a toll on the paint and the logo would just be bare plastic. The person could have repainted the wording, but not the sun itself.
Look at page 2 of the '77 TT brochure. The 12 1/2 MC (which is on the left), has the faded stripe from willow green to a yellow to light green.
2007 T-286SR Cherry/Granola, #6236, original owner, current mileage: 9473.8 (as of 6/18/21)
1997 T-2653 Blue Denim, #5471
1979 12 1/2' MC, Beige & Avocado, #4639
Past Sunlines: '97 T-2653 #5089, '94 T-2251, '86 T-1550, '94 T-2363, '98 T-270SR