I had a sudden thought today and went up into the loft this evening and got out my 3781 to check the knob for turning the turntable base. It is about 3/4 of an inch in diameter and smaller and finer than the one on my 3761 crane. I would suggest that what you have in the photo is an early and rather battered knob for turning the turntable on a 3761 crane and not for a 3781 as I suggested at first. I had thought that Playmobil would have used a common knob for these two models but it seems not. Certainly, the weight which has to be turned on the crane (especially if it has a load on the hook) is much more than on the fire engine and so a larger and easier to grasp knob was used.
I had also had a look into this today and did notice that the yellow mobile cranes on ebay had the larger knob on them, rather than the smaller ones on my fire engines. This would also make sense because I have a few other knob /winding parts from the yellow mobile crane set, so in with them it shall go.
Did you know that the green mobile crane from mega construction set 4084 doesn't have mechanism for turning the turntable - they expect you just to turn it yourself, even though they haven't changed the casting of the turntable as it still has a hole to slot the knob into whilst on the move
