Got my first ever Parker "51" vac and replaced my second ever vac diaphragm yesterday. The hood didn't want to come off so easily. Though I would have liked to be able to inspect my work from the inside when I installed the diaphragm, I didn't have any pressing need to remove it, so I figured I'd apply patience and persistence to eventually get it off another day. My question is about how far the nib sticks out of the hood. This 51's nib doesn't extend very far out of the shell, only about 1.1mm. My 51 aeros all seem to extend about 2mm. The difference when using this pen is noticeable to me--it looks like the shell is close to the page. It's the only reason I noticed in the first place.
Now, in the aeros, I just insert the nib as far as it will go and thread on the hood all the way. Is this considered "adjustable" on the "51" vacs? If not, is it a trait of some specific vintages? This pen is from 1Q 1945.