PART NO.  SPL.PAGE  DESCRIPTION                                                           
PFS310    TC.R.1    Nut for screw (14) -(panel, front wheel arches detachable, body shell,
    Tourer, Twin Cam) Com.(C)592
PFS310    TC.R.4    Nut for screw (14) -(panel, front wheel arches detachable, body shell,
    Coupe, Twin Cam) Com.(C)594

The Twin Cam SPL lists these part numbers for attaching front inner wing vent panels:
PJZ1003 Screw fixing panel (14 ea) -(#10-32-UNF x 3/8 long)
PFS310 Nut for screw (14 ea)

U-nuts go on the edge of the removable panel, which in turn goes inboard of the inner fender, with the screws installed from the outside. When you remove the screws, the U-nuts stay attached to the removable panel. No concern for flush fit or sealing, as the removable panels are louvered for ventilation anyway.
In the USA we call the press-on nut Tinnerman fastener, which is a brand name that has come to be used commonly for almost any kind of sheet metal clip. The more generic name is U-Nut for a clip-on nut that will accept a threaded fastener. Part number may have changed later to GHF713, which is another Tinnerman type clip-on nut that accepts the same screw (but might be for thicker sheet metal).

The original screws were Pan head, Phillips drive, #10 X 3/8 inch coarse thread screws with a truncated (blunt) tip end. The blunt nose is a safety feature to protect hands and arms when working in the confined space of the engine bay.

A commonly supplied modern screw type would be the Pan head, Phillips drive, #10 X 3/8 inch sheet metal screw (Type A thread). This is nearly identical, except it has a sharp point with potential of causing personal injury when working in close quarters.

Thanks to a note from Aaron Coolidge, this screw is identified as Type B Self Tapping Screw, #10-16 straight thread with tapered nose and blunt tip. It has a larger root diameter and finer pitch than the common Type A pointed tapping screw (which would be #10 with 12-tpi), so the mating captive nut will also need to match the Type B thread form. These screws are apparently common with aircraft parts supplies, but also available from McMaster-Carr here in the States. https://www.mcmaster.com/90056A240

More information at: LOUVERED PANELS For Inner Fenders - TC-117