Ah just teasing ya Steve, thanks for the answer.
Ale, I think you are right about materials. I found this info from
http://www.jacop.net/gear.html
1962 Fender Jazz, a.k.a. the "Bass of Doom," SN 64437
Like the fate of a mythic hero's mighty weapon, the original condition and final resting place of the world's most famous fretless are shrouded in mystery. Its legendary tone was well documented through every era of Jaco's career, and he himself told several versions of the tale.
According to Bill Milkowski's August '84 Guitar Player cover story, the '62 Jazz was already fretless when Jaco bought it in Florida for $90. Upon meeting Kaufman in 1978, Jaco told him he removed the frets himself with a butter knife and filled in the slots and missing fingerboard chunks with Plastic Wood, followed by several brushed-on coats of Petite's Poly-Poxy. Kaufman's first job for Jaco was to replace the peeling epoxy, which he did by using his own method of pouring on the epoxy in one treatment and shaping it with a rasp. According to Kaufman, Jaco left it in New York's Central Park shortly before his death. It hasn't been seen since.
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Just to add a little story. In 1993 I'm walking to MSG for the train and pass through 53rd St where all the music stores are. I passed a store with a fretless Fender Jazz and a notable large letter P between the pickups. I recognized it immediately and ran in the store. They said it was Jaco's, they got it from a student of his and wanted $25,000. I called Bass Player magazine to tell them and spoke with one of their editors. he said he'd already heard of it and they were planning a story. Not sure which bass it was but it wasn't the legendary '62.
BTW here is the Jaco video. He was already going downhill badly and it shows.
https://www.youtube.com/watch?v=pZSa9niKKE4
He talks about not practicing with the fretless at 9:28