Thanks for the info, that's exactly what I needed to know. I've removed and replaced a few and feel comfortable with that but I'm not too sure about modifying a box to fit a different diameter barrel. I want to be sure I have the right one and put it in just once.

Is there a way to look at a Waterman lever box without the original barrel and determine if it came from a 52 or from a 1/2 size barrel?

Thanks for the responses. I remember now reading something Ron wrote about this a few years ago. I should have looked back at his blog before I asked the question. I agree with the second primary rule of pen repair "replace only when it's impossible to repair". I'm sending it to Ron.

That is a good point. I'd bet you're right about the threads. And, yes it is marbled. The second digit appears to be rubbed out from posting.

I have a Burgundy junior lockdown with a 4 date code that the cap will not screw on. I have switched the cap with a similar Grey model and that cap screws onto the Burgundy barrel well. The Burgundy cap will barely catch the threads on the Grey barrel. I removed the section and the cap screws right on. I've checked the widths of both caps and they are the same. The sections are the same also. I have measured from the cap edge to the inner cap and it is the same on both caps. It seems to me that the inner cap is the problem but I can't figure it out. Any suggestions? Is it a relatively straight forward fix or should I just get a new cap?

Anybody have a forest green shell for a Parker 51 aerometric (48 date code, fully threaded)?