Astoria changed the cap thread over time: depending on the production period it matches Montblanc's or differs from it. The simplest test is therefore to screw on a Montblanc cap of the right size. If it fits, the prospects of finding a replacement improve considerably, because a Montblanc cap without an imprint is, by collectors' reckoning, about ten times easier to find than an original Astoria cap.
On safety pens the cap is the part that breaks first, and thus the most sought-after spare part of all. An alternative to searching is repair: experienced restorers turn missing cap lips from original material on the lathe and fit them to the existing cap almost invisibly. With a cap whose logo top is intact, this route is nearly always worthwhile.