Did some more research
Some interesting reading that missing engine no for a 70 was the norm?
https://forum.mustang.org.au/index.php?topic=911.0
I realize that your searching for answers but I think your stretching hope a bit with this one.
Please understand that it was a Federal requirement in this country to have the engine and transmission stamped on every US sold car in 1970 and the Ozzys may be reflecting on and including in their experience the 351 Cleveland's they had installed in their Australian Ford products. Sure plenty of their Mustangs have had transplanted engines from local source over the years. Don't see anything in that thread other than one or so opinions from people in a country without allot of cars or opportunities to see allot of 1970 cars,
Notice that some of them are making reference to finding them at the front of the engine. This is the same end of the engines we find our assembly dates on engines built in this country. In another post someone claims that the US didn't require VIN's on all engines until 1970 which is incorrect it was 1968. Thought we had a thread discussing the 1968 law/requirement and when Ford instituted the practice (it was before the law went into affect). If I can't find that thread I'll assemble the references so that it is present here for members and lurkers to read.
In this case IMHO the site and members are not very familiar with US practices nor Federal requirements. A number of our members post on a regular basis on that site also and I check it out ever day or so.