assume you mean alternative in size? ...... the nib for said pen is quite narrow - someone may be able to suggest a specific brand nib that is the equivalent in size. I'd suggest a Swan No. 1 is quite close (it may be just a tad wider across the shoulder) - if not very nearly the same in width to the Blackbird - but assuming you are missing a nib for your Blackbird pen then you are going to need to source a replacement however you jump. The easiest option might well be to use ebay for a donor Blackbird with a half decent nib, or perhaps just wait for a B. nib to surface - that is unless you have a stock of a few spare small nibs that you might use to assess for suitable spare. Your pen would regain its originality if you are able to source the correct Blackbird nib.
Stephen Hull's books are recommended very highly if you collect British pens, wherever in the world you are, and I sense his offering on M.T. is going to be the best yet, and may well outshine his other volumes - if only because of what seems to be a high level of collector interest in this make.
Probably because I'm rather old and thick (but only at times you understand) - but I've obviously missed the point of Cob's words ..... "Clearly there was a determined effort to market them in the Netherlands." What have I missed within the images/wording above that I should have seen and that would have made those words 'clear'?