Last May I was flying from Istanbul SAW and saw this ACT Cargo 747 shortly before take-off. It's clearly under maintenance, but without wingtips of a regular 747-400. Does anyone know which aircraft this exactly is? I don't remember there being a 747-400D without full upper deck.
I don't know why you'd remove them for maint unless maybe it's a major check.
But when undergoing maint, the airframe doesn't need the winglet so it could well be the part was removed to help out a sistership with a spare for damage repair.
We did this quite frequently: when there was an unexpected AOG we just went over to the hangar and canibalized a sistership to get the other back flying ASAP. Different type but it doesn't really matter I think.
A 747-400 can fly with three different wing configs btw.
1. Standard with wingtip extension and winglets
2. Same as above but without winglet (Boeings LCF does this for example)
3. Winglet and wingtip extension removed - the missing extension also reduces the number of outer wing LE slat segments from 6 to 5. You get a "classic" wing or a "747-400D"
I may add that 3. might only work the other way round: according to certification a -400D has provisions to install the extension and winglet, but it's not on and off.