My Sunburst 1973 Precision - which I got brand new for Christmas 1973 - came with the ashtrays/covers attached, with flatwound strings and a foam mute under the bridge cover.
There have been many stories as to why the ashtrays/covers were put on there, and what function they served, so I shall now explain the whys and wherefores as have been explained to me over the years...
Firstly, as they were
never grounded to the the wiring loom, anyone who says the ashtrays were there a for screening purposes is wrong - looks like they should be, but Fender never wired them up, so they weren't.
Next we should consider how the Bass was played in the 50s and early 60s - basically (sic) you plucked the strings with your thumb - there's a scene in the first ever James Bond movie - Dr. No (1963) - set in a beach bar in Jamaica, where you'll see a perfect example of this... (Jamaica Beach Bar, Red Stripe beer, beautiful ladies..... sorry, I got distracted

)
Now if you pluck with your thumb, you need to anchor your right (plucking) hand on the pickguard and, if you recall a '60s Fender Bass, the finger rest is conveniently located under the G string at a slight angle - in fact the perfect angle to anchor your fingers and give you the leverage you need when plucking with your thumb.....
However, if you do this, then your right forearm is going to have to lay across the strings, so the pickup cover is there so your jacket sleeve and cufflink (you
are wearing a tux aren't you?) doesn't foul/catch the strings.
To match the pickup cover, we need something similar over the bridge (which is an ugly piece of industrial bent metal that rusted anyway

) and it is a convenient place to attach the foam mute over the flatwound strings to give you that dull thud we all know so well...