In reply to:
The nib on the Moonman S1 has no grade marking, and the item was originally listed as having an “EF/F” nib — except there was no selectable option at all. When I asked the seller for clarification, he/she said there was only one nib width, that being “0.5mm”, which would make it a “Chinese F”.
I swapped the Majohn A1 and Pilot Capless nib assemblies into the same (clipless Majohn A1) pen body, and used the same converter with the same fill of ink, for (2a) and (3a), as well as for (2b) and (3b), to minimise any unconscious differences that could arise from the the weight balance of the pen.
Pelikan 4001 Brilliant Black is supposed to be dry-flowing, while Jacques Herbin Noir Abyssal is supposed to be wet-flowing. However, it just seemed the former wanted to misbehave today on this sheet of paper; and the latter actually seem to be more dry than the former coming out of the Moonman S1, putting down crisper but shadier lines.