I'm not a Dunhill owner and don't have calipers, but this spurred my curiosity. Are you wanting to measure the spot for authenticating or dating pipes, or for some other reason? Idle curiosity, but thought I'd ask.
Jay I think some other guys have that market cornered.
I bought a pipe, mso.
It's a 9k gold banded Astleys Panel. Les Wood did the band and its hallmarked 1993. So much scuttlebutt about Astleys suppliers indicates Dunhill, Ashton, and others.
To my eye the shaping points to Dunhill.
The top of the stem has the Astleys logo, but the underside has a "black dot" where a round inlay would go. It measures 3.4mm approximately.
I measured the inlay on a 1986 Ashton and it's more or less the same.
I'm now thinking it was more likely a Bill Taylor creation.