Montblanc ballpoints of the 1960s already took the then-new large-capacity refill, but its writing tip was of smaller diameter than today's. In the 1970s the tip diameter was increased for the sake of a better writing performance. The front section of the older bodies therefore has a collar that narrows the bore, so that a current refill will go in only a few millimetres. For about ten years after the changeover Montblanc supplied replacement front sections free of charge, then stopped, on the assumption that no old ballpoints were still in use. That is why pens with too narrow a bore are surfacing again today, having lain unused for decades.
Drilling the bore out is not to be recommended. The early plastics have lost most of their plasticiser and have grown brittle, and the surprisingly long brass core carries the frictional heat, so that the material distorts and usually cracks. The better answer is an adapter for a thick Euro-standard refill from Schmidt or Schneider: the writing tip of these fits the old front sections, while the rear end of the refill has to be adapted. Adapters of this kind used to be made up by collectors such as Jürgen Kuhse in Cologne for a small fee. The same thick Euro-standard refill can also be used in the so-called Ballograph Montblanc ballpoints of the 1950s, if its plastic end is cut off and filed round.