The single piece shafts, I have Chris Cole pieces in mine including the 1 piece shaft do not have any radial (my "in and out" correctly stated) play in it at all, none.
If it's not designed to be pinned then what is the hole for? Does it only hold the splined coupler onto the driven gear stack shaft and the input shaft is free to pull out? Is that what we are experiencing? The point is I think it would be very helpful if one of you guys in Europe that knows for sure would let him/us know. I would hate to have a good gearbox damaged for lack of complete information. I know mine didn't move more that a .1" or two in and out when it had a two piece shaft in it. It did have a bit of lateral play. These are pretty old gearboxes at this point and I am sure there are different configurations as well as many different states of wear. How's that old adage go. "Plan for the worst, hope for the best"
Another consideration is if the shaft jams in the out position it may/can load the thrust bearings in the engine and result in a worst situation. That would be a one in a lot type of failure but I would want to be certain if it was mine.
What happened to lock it in place after it was pulled out? Possibly a piece ( it may have had since broken into 2 or more pieces) of the original pin dropped back into the hole and thus it won't move anymore. The question is really what is going on in there and if it had a pin in it what happened to the pieces if it broke? All important information, especially if it is a good running gearbox. Like I said, it would be a shame to damage it unnecessarily.
Maybe call Chris Cole and ask?