Hi Ben
The early Heuer Quartz divers were fitted with an ESA 536.121 and I stand corrected, but believe these were used up until the Tag takeover when we see the ETA movements. This was around 1986 and the first series to use the ETA were the Heuer 1000 (no TAG on the dial) - see number 4 in the picture.
The 844 (automatic) and 8440 (quartz) were the first of the Heuer divers (referred to as the 1000 series). They were made in France and have a unique dial with 24 hour numerals (12 to 24 in red) and cathedral hands. (number 2 is an 844)
You can read the history of the 1000 series on Davids blog calibre11.com.
It would be great if you post a picture of the dial.
regards
Paul