I have been through a long process of restoring a vintage Onoto 6233. It took me longer than expected mainly due to my trial & error learning process. I had to order my spares twice through Roger Wolfe's custompenparts.co.uk. I also am most grateful to Richard Binder for his instructions, both from his book and website.
In spite of this guidance, I am not entirely at ease with this plunger mechanism, which I find in Onoto pens unnecessarily complicated in design. I replaced the rod, which I had broken in my learning process and followed Roger Wolfe's instructions in identifying the proper position for drilling a hole through the rod. With the section opened and the plunger in working order, I hear a sharp pop when the pluger reaches the end position, therefore I believe than air tightness is not an issue. What I am not too sure about is the correct position of the plunger tip, once the plunger knob is screwed in. Should it rest against the feed assembly or should it leave a gap? I've read somewhere that the Onotos have some safety feature to prevent leakages and that one needs to partly unscrew the plunger knob when writing.
I hope some Onoto expert is reading my message and willing to help.