Hi Jim,
Normally it took around 3 months from the Briggs plant to the customer in Australia, that being Scott Bonnar. Now being that your engine was built in early October 1975 that would've made it late December for delivery to Thebarton in Adelaide. Then one must consider the Christmas Close down from around 23 of December and over most of January, things would've been slow at that time of year, thus I'd say the mower would've been made (assuming that engine is the original unit )around early February 1976.
I must say the photos are not that good lighting wise and it could still be a Alpine Green unit as the sun is making it rather confusing, but one thing is for sure it should have a black chain case and shield for that era and not all green.
Pentagon Logo on the catcher is correct.
Cheers,
BB.
