Can anyone throw any light on this Genuine Bellett Engine Number...?

Never seen one like that Kerry.
The numbers etc seem all over the place, not in line, different sizes. Maybe another state based Reg dept on an un numbered block like the "VP" numbers the Police used to issue .

When I first saw the number I thought it was unusual. G150 denotes engine size of course. Isuzu engines can be found in various machinery. For example a forklift or a mobile crane. The manufacturer may have called for tenders to supply and Isuzu was the successful bidder. This is a bare engine. No ancillaries. Then the manufacturer, for their own identification, had the numbers and letters stamped accordingly. If it was in a forklift say, a hydraulic pump would be mounted on the front of the engine instead of the timing cover. LPG fitted. Alternator with a vacuum pump for the brakes. Al of these would be in conjunction with the engine number and ID./ chassis or frame number.
How's that Quiz master Kerry.

Dave.

Could mean anything. They stamped them with all sorts of weird combinations.
Some think G160/161 engines started at block number 300001, but there's an ex race engine kicking around that is clearly stamped "G160 - 24".....

I have in my parts stash a replacement part new G161 block. It has a blank engine number space. I guess you could have stamped your original number from the block with the rod through the side, or you could samp anything you liked, as long as you notified the rego people.
Cheers, Matt.