But, yeah, Jefferies did do some SF designs outside of Trek, though that particular instance got unofficially "assimilated" later.
I agree – I owned the "UFO Mystery Ship" version. But I was thinking in larger terms. He helped to visually create an IP that has stood the test of time – almost half a century! I can only imagine what he might have come up with if he had been set free on another project the size of Star Trek.
You're probably right. I was unclear whether they were actually cargo or fuel pods. I think someone interpreted them as fuel pods that were dropped along the journey.
Whatever they were, MJ's sketches show that they were modules and there could be more than we saw on screen. It's that level of design and execution that made Jefferies' work so memorable. Form was built on (fictional) function.