Flap 25 was a classic limit, introduced to take the load off of the flap track fwd. fuse pins that had the odd habit of breaking.
IIRC Pan-Am were the first to introduce it, and later a Boeing recommendation due to the failures, and yes, we had a couple of failures too
Must admit I would have expected to see more of the slot between the mid flap and aft flap which only begins to open up as the flaps translate from 20 to 25 units. Must be the angle it's viewed from.
