Heya Captive_Ball. You said the magic word (Darling/Jubilee), which means that I will impetuously insert myself into the conversation, like a needy genie.
Spacelab stripped down? Mmmm...I'm not sure I'd go that far. It's different (A LITTLE! ). ...but stripped down? From what I can tell, the only difference in the main playfield elements is that on Darling/Jubilee there's a sideways gate (located just above the captive ball loop between the two pops), which was removed on Sky/Spacelab. *shrugs* The other difference is that you have a traditional "Italian bottom" complete with inlanes (which seems to be a rarity on Williams titles from this era) on Sky/Spacelab, whereas on Darling/Jubilee you have those kick-out holes at the base of the flippers which do not allow for cradling. So, those are a couple of things removed/changed, but I dare say that the art package on Sky/Spacelab is pretty darn cool. I love the fact that they made a three-dimensional rocket out of the captive ball loop. Also, those pop bumper caps (which are in-tact[!??!] on this seller's listing) are probably worth as much as the machine. Ha-ha!
Anyhow, I've met my Darling/Jubilee diatribe quota for the day. Hope this finds you well, CB!