OK this is weird, I've been lurking here for a while but never had anything constructive to post. I also have a 500-4 that I've had for 20 years, and never seen another one, on line or in person then 2 different posts about them in the same day...strange.
http://www.carvinmuseum.com/decade/74-basses.html has some good info
"In the 1960's and early 1970's, Carvin bought necks from Höfner, and used them on their own guitars and basses. Höfner was founded in Schönbach, Germany in 1887 by master luthier Karl Höfner, and was the largest manufacturer of stringed and fretted instruments in Germany in the late 19th and early 20th centuries. His two sons, Josef and Walter, joined their father's company in 1919 and 1921, and they successfully expanded Höfner's worldwide market, enabling them to survive the years of recovery following World War II.
Carvin's relationship with Höfner began in the mid-1960's, and would last until the late 1980's. Carvin guitars and basses from 1964 until 1978 had bolt on Höfner necks, and the SH225 was made by Höfner and sold under the Carvin name. The Höfner 500/4 bass above is remarkably similar to the Carvin AB45. Obviously, Carvin got the body as well as the neck from Höfner, as the body shape, F holes , binding and finish are identical. Note that the headstock logos are also the same. The significant difference is that Carvin used their own electronics and pickups in the AB45."
my bass. When I got it, the PO had cut it up to put active Bartolini "J-bass" pickups in it, and of course screwed it all up. It's better now, but only slightly original, none of the electronics were there when I got mine so all that's been replaced now. I searched for years for any info about it, and only found anything in the last couple, like the Carvin connection. As far as Ovation having similar stuff, I know Teisco, Kawai and Aria all made a nearly-identical bass, the O's might just be re-branded ones since Matsumoku/Kawai made dozens of brands in the same factory.
http://www.matsumoku.org/models/aria/hollow/sem/semi.html