Montblanc has altered details of its writing instruments again and again, and the 149 is no exception; the same goes for the other Meisterstück models. Among the points reworked more than once is the cap thread, with the result that caps from different production periods do not necessarily fit one another despite carrying the same model number.
This is not a defect, not a trace of repair and not a sign of a fake, but a normal consequence of so long a production run. In practice it means that a replacement cap has to come from the same period as the pen. Buy a cap blind and you risk its not engaging at all, or only stiffly; force a mismatched thread home and you will damage cap and barrel for good.
Collectors' surveys exist for the 149 that list the changes to nib, feed, cap ring, piston mechanism and threads year by year. They are the most reliable aid there is for dating an example and finding parts that fit.