I believe I’ve written before about the colossal stacks of old car brochures I was sent by a kind reader who perhaps secretly wants my body to be found under piles of automotive paraphernalia one day, and, lucky you, I’m about to do it again. In fact, last time I wrote about some , and it turns out I’m at it again, with another bit of Rabbit brochure weirdness. This time, it’s about a feature VW thought was important enough to highlight in callout in a diagram, but it’s also a feature that would baffle modern car buyers.

Here’s the diagram in question; see if you can guess what I’m talking about?

Sure, a bunch of these barely seem like things worth calling out today—hidden spare tire? Carpeted door panels? Hand-fitted headliner in trunk? But I think the one most likely to make people wonder “why the fuck would I want that?” has to be the “.”

I like to imagine someone looking for a new car back in 1977 really grilling their AMC dealer about the Gremlin, brushing off the salesperson’s excitement at the Gremlin’s new front end and saying “I don’t know. How about the battery? If I hold it up to the light, will it glow? I just got back from the VW dealer, and let me tell you, their battery was letting through all kinds of light. Like a fucking cathedral window. Can you beat that?”

So, what the hell is going on here? Why is VW pointing out the battery is translucent? Why would you even want a translucent battery?

The reason has to be for something that is just not a big deal in modern cars or with modern batteries: fluid level checking. There was a time when batteries required maintenance in the form of replenishing the distilled water in the battery’s cells, which would evaporate or be outgassed with use.

So, with this in mind, a translucent battery where you could see the fluid levels in the cells at a glance would be handy.

What’s puzzling, though, is that sealed, closed-system batteries that did not require adding water . I would have thought that by 1977 all major automakers would have been using maintenance-free batteries? So far I haven’t been able to find out what kind of battery these were originally sourced with, save for the fact that they were translucent, which may imply they needed some maintenance.

As an aside, that first battery to not require any maintenance was called the Delco-Remy Freedom Battery, and it really was quite an under-appreciated revolution. Well, maybe it wasn’t entirely unappreciated, since there does seem to have been a commemorative Jim Beam Delco Freedom Battery decanter sold:

Who among us hasn’t wanted to swill Jim Beam from a car battery?