Yes, it’ the extra thin neck refill. The 225 won’t fit, its tip is the right size, but the tube right behind it is about 0,5 mm too thick.
My current solution is a modified A3 plastic refill: gently shaved the tube with a box cutter for about 1 cm behind the flare of the tip, shortened in the back by about 5 mm and replaced the original spring with a longer one as the stop on the A3 is further back than the expansion of the original refill.
