Beach Boys Animated Special.

I found this post on the eBay Community, apparently by the owner of the pieces who was looking for more information. There are two more images different from the ones you've found community.ebay.com/t5/Collectibles-Art/Need-Info-Re-Vintage-70s-Cels-of-The-Beach-Boys-as-Cartoon/td-p/28767966

She says she got them from a family member who got them from a flea market, and that the seller had no additional info. She also identifies them as 17-1/2 x 9 in hand-painted cels.

Now for some speculation on my part. To me, the art style doesn't say 70s at all; it looks modern. I feel like there is inspiration drawn from both "Popeye" and "DuckTales" and meant to evoke an overall retro style without imitating any specific time period. It's also very neat and clean, much more polished than I would expect if it were concept art for an unproduced cartoon in early planning stages, and they are in surprisingly good condition if they really were cels from the 70s, as cels degrade quite easily if not kept in ideal storage conditions. For these reasons, I think it may be fan art created fairly recently. How it would have ended up in a Flea Market, I don't know. It would be pretty impressive if a fan went through all the trouble to hand paint them as well, but entirely possible. Keep in mind, this is all just my own conjecture. I would really be interested in finding out who the artist is, whether it is fan art or not.