My 1965 Jazz had both covers in place when I bought it in 1966. I took the neck pickup off because that's pretty much the spot where I like to pluck the strings and it was in the way. In 1965 the "ashtray" cover with the giant F on it had a foam strip glued to the inside of the cover. When the cover was in place, that open-cell foam strip contacted the bass strings and acted as a mute. Evidently Fender was still trying to make the electric bass sound more like a stand-up bass and over the years they used different muting methods to try to achieve that.
What those string mutes really did was kill the sustain on the strings. I liked that big ashtray cover at the time but didn't like the mutes. So I cut and scrubbed that foam strip off. Don't know that I ever got it perfect, but it didn't really matter. I removed enough so that nothing muted the strings and, since it was on the underside of the ashtray, you couldn't see that part anyhow. I ended up playing with the center cover removed and the bridge "ashtray" cover in place right up until my bass was stolen in 1976 when the house we were staying in was burglarized (also stole my amp and mics).
Below is a shot of me playing my '65 back in about 1972. We typically played 5-nights a week and my bass is pretty smudged up here...sorry 'bout that:
View attachment 3895201
In 1985, when my current bass was traded in towards another purchase at the music store where I was working, The covers were inside the case and still in their plastic bags - they were never mounted. The fellow who took in the bass immediately removed them from the bass case and put them into our accessory case by the register to sell them separately. So they didn't come with the covers a month later when I was allowed to buy it. Actually that sounds bad, it was actually good. The owner wanted to try to sell it at retail for a month before he sold it to me. It didn't sell in that longest month I ever lived through and he gave me a very good price on it at the end of that month - very grateful to him for that.
I do not use the covers on my current jazz at all and never will. The pickup covers/protectors drastically inhibit where you can pluck the string and as a result, they drastically reduce the variation in sounds that you can get out of it. The crude basics or it are that if you pluck the strings up by the bottom of your neck you will get a warmer and darker sound. If you pluck them back by the bridge, you will get a very bright and percussive sound. If you want to make your Jazz growl, dig into the strings over the bridge pickup. So, knowing what I do know about how to get different sounds out of my Jazz Bass, I would never use the covers they came with. They are just too limiting to my choices of sounds.
And below is my current bass, which is a 1982 Jazz with the same neck profile that my '65 had. I'm going for the warmer/darker sound plucking the strings as you see here. Notice I have lots of choices on where to pluck the strings to get different timbres to the sound from them.
View attachment 3895195
