Vintage 30 China vs UK (Graphs)

[MirrorProfiles]

I think you might be right.
The dust cap may have a lot to do with how guitar speakers sound, diameter and material stiffness directly affect high frequencies.

Good read :

Dust Caps

The speaker dust cap covers and protects the voice coil and internal components within a speaker. It is usually made of a soft mesh type material, although some speakers have aluminum or plastic dust...

www.guitarrecordinglounge.com

EDIT:
Since Celestions dust caps haven't changed in decades, I wonder if the amount of applied glue and type of glue have a direct affect on the brightness of the speaker.
I may do a little destructive science to find out.

I remember this being a topic of discussion a few years ago with Nolly's deep dive into V30's.

IIRC Supposedly there was a chemical in the glue that was banned in China but allowed in the UK (or vice versa) so they had to be different. The glue absolutely has an effect though if you put the mic over where the glue blobs are it sounds noticeably more rolled off than the other side. I'm sure the application and amount of glue would affect the whole speaker.

Something else that can in turn have an effect is the orientation of how the speakers are mounted in the cab - purely for how we typically put the mic halfway up the vertical axis in the centre of the speaker and move left or right. Mesa mounts their speakers at a 45º angle so the glue blobs are in a slightly different position to how they'd be in a Marshall style cab. In a Marshall cab the glue blobs would make a 90º right angle point towards the ground and the side - in a Mesa they're shooting off diagonally.

 

[James Freeman]

if you put the mic over where the glue blobs are it sounds noticeably more rolled off than the other side.

Hmm.
According to the linked article a stiffer dust caps attenuates the high frequencies and softer dust cap extends them, and possibly exactly at the wavelength of the dust cap diameter as @DieSchmalle noticed.

So the less peaky 'UK' sounding Celestion might actually have a harder dust cap glue and/or stiffer dust cap?

Look at Figure 0.16 in the linked article.