I have a number of 3776 pens with newer and older (round breather hole) nibs. Nibs also come in two sizes 6.0 and 6.5mm. Have not determined if the change in nib size coincided with the change in breather hole.
My largest nib is a MEDIUM. Smallest is an UF. The UF and FINE provides feedback. The MEDIUM does not. Might be due to my writing style. I also suspect Japanese nibs were designed to mostly write kanji and kana. It may be anecdotal that the newer model nibs provide more feedback. To me the older ones are much smoother and perform slightly better. BTW, the older nibs come with ebonite feeds. Again, have not determined if the nibs and diameters wrre changed at the same time as ebonite feeds were phased out.
Suggestion: If you see any of the older models for sale, except for the plain Jane black, go for it.
Formerly Ryojusen Pens
The oldest and largest buyer and seller of vintage Japanese pens in America.
Member: Pen Collectors of America & Fuente, THE Japanese Pen Collectors Club