The Mentmore Diploma first appeared in 1934 and was updated to a semi hooded version, this pen, in 1948, presumably as a response to the hooded nib of the Parker 51.
The pen is a typical English button filler and holds no real surprises in the restoration process.
With gentle heat, I use a hairdrier, the nib section can be removed, it's a right hand thread. Care should be taken when using a drift and knockout block to remove the nib and feed as the hood shape and size can render it succeptible to chipping
The barrel is generously proportioned, a new size 19 ink sac, quite large, will hold a good amount of ink and still allow the pressure bar to be fed in from the top when reassembling.
To remove the filler button it is best not to grip the button with bthin nosed pliers but use them to exert pressure on the underside of the button lip to pull it out. My advice is to not be tempted to leave the button and pressure bar in place and attempt to do the whole job from the nib end. The section is a screw fit and it ia almost impossibe to tighten it without twisting the pressure bar and perhaps damaging the new ink sac.
With the section reattached and the button removed the pressure bar can be fed in from the top. It can be a little fiddly to get the right angle when inserting the bar, it is important to orient it on the correct side of the sac. Finally the button can be firmly pushed back int position.
Immensely helpful as it has enabled me to restore a very congested Mentmore 69. I haven’t been able to find anything about that particular pen but it seems very similar to the Diplomat but with a different nib holder that unscrews separately. The advice on fitting a new sac helped. I found that cutting the length of the new sac so that the top ends as the bar curves in at the button end works and makes sense. It writes beautifully, very little flex with a smooth fine line.
Many thanks
John