I seem fated to acquire pens without caps - attached is picture of another, probably identical model to my earlier capless example, and this one again with the over/under nib feed, although the chasing looks more like spiralling chevrons, and barrel length is 81 mm. The pen came to me in the accompanying box, although unsure if it would have been the original for this pen or not.
The blind cap carries some marks which hopefully the experts here will be able to clarify, as I don't have Stephen Hull's book.
Two pairs of marks.......... the first shows what is just possibly an earlier owner's efforts to show their initials, and looks to read upper case A H - and the other clearer marks show what appears to be 3 r 19, or something very similar.
Sorry the pix aren't too good, but hopefully legible