It is from an Acacia Ridge assembled HK Belmont, Kingswood or Premier.

They are all the same engines, Monaro is identical to Belmont, Premier, Kingswood, Brougham and GTS.

The number fits somewhere after HK commercial production and before Monaro and Brougham production at Acacia Ridge, so roughly April-May. So it would have originally come from a Belmont, Kingswood or Premier, sedan, wagon, ute or van.

At Acacia Ridge, HK Monaro and Brougham V8 engine numbers started ballpark in the 800's. GTS probably low 900's. You can see how few Small Block Chevy design engined cars Acacia Ridge produced, that by July 1968 they'd only just got to their 900th, whereas Pagewood for example was in the high 2000's by that stage and accelerated massively once GTS production started. Conversely my red Acacia Ridge GTS327 didn't get assembled until October 1968 and its number is still lower than 1300. Pagewood at that time was up in the multiple 1000's, by the end of HK they were over 18,000 V8 engine cars.

I already said it cannot be from a Monaro, too early by at least 115 numbers for Monaro and around 175-215 for GTS. Even if it was from a Monaro big deal, a Monaro is a Kingswood with two doors, and as I said a 307 is the same regardless of what it's in - they are all the same for Belmont, Kingswood, Monaro, GTS, Premier or Brougham, ute, van, sedan, wagon, coupe or Brougham. The only difference between any 307's is the flywheel and bellhousing for manual and flex plate for auto, plus the 4 x different carbies for manual/auto with/without A/C.

There is documentation showing all this but it is private, the result of 30+ years of research inspecting and recording cars. Holden would have no idea, they wouldn't even have records of engine numbers for these and they gt rid of all their original documentation. They never recorded engine numbers in cars - it was up to the dealer to send that info back with the delivery paperwork, but many cars never had that data entered into the IBM 360 mainframe. Limited reports out of the IBM were done in the 70's for service/warranty and were microfiched when GMH changed to a new warranty system mid 1978, but to date the HK-HG report has never surfaced.