For those of you who've asked, I've been away in Hong Kong and Melbourne. Pleased to say though that the watch collection now has three more additions: a neat 6309-7040, a 6118-8000, and something Swiss.
Here is the noticibly larger -7040 (built July 1984) against my older everyday rat-729A (built October 1986):
While the case is obviously larger, the dial and bezel appear to be the same size. Comparing them though, the bezel on the -7040 is about 0.5mm larger - I always thought them the same.
Apart from case dimensions, the most obvious differences are the face markings:
The bezel and 'Seiko' fonts appear the same, as do the hands.
The crown protectors:
Dial details though vary:
-7040 says '6309-704X T MOV'T AND DIAL JAPAN CASED HONG KONG'.
Case back, with same tsunami, but obvious case shape difference:
Again case shape and crown protector variations visible (the -729A looks as if the crown protection could take someone's eye out):
After a few minutes under the photo lamps, the difference in their lumescence can be seen (note earlier watch's decay and lack of hour markers in the dark):
The -7040 will remain a collector item and the -729A the everyday biking and skiing device.
(Hopefully, I'll be able and post the results of another Tokyo watchshop adventure soon too. Thanks for the appreciative comments.)