Re: format difference between a 4040 and a 1541

I've just formatted a floppy with a "1540" (1541 drive with a E000 rom
image of the original 1540).
Header to Sector gap as expected is only 8 raw bytes:

   ; header
   gcr 08
   begin-checksum
      checksum 16
      ; sector
      gcr 00
      ; track
      gcr 12
      ; id2
      gcr 30
      ; id1
      gcr 34
   end-checksum
   gcr 0f
   gcr 0f
   ; Trk 18 Sec 0
   bytes 55 55 55 55 55 55 55 52 bf
   bits 111

The end of sector gap is one byte shorter on the lowest three zones
and the same size as the 1541-05 on the last clock zone (checked on
track 35), that's the same hardware where I installed the 1540 eprom
so the spindle speed should be the same for the two floppies I've
formatted.

Another interesting thing I'm observing is that  the Header gap on a
4040 formatted-but-never-overwritten sector is really 9 raw bytes and
becomes 10 raw bytes on overwritten sectors, which is really
interesting. Does anyone know where in the 4040 code this difference
happens?
A 1541-05 and the 1540 when rewriting headers seems to "add" half a
byte to header's gap by spoiling the first bits of the next sync mark.
It seems to me that 1541-05 and 4040 formats are completely
"cross-writable" if I'm not overlooking something. On the other hand,
1540 header gap seems too short.

Best regards
Frank IZ8DWF