The SCPL was an ICAO license, not an Australian invention, the criteria for getting an SCPL were somewhat less than an ATPL.
You are quite correct LS. At that time two of the requirements for a Class 1 ATPL were that you had to be command checked on an aircraft over 12500 lbs and CURRENTLY flying for an Australian Airline.
The latter requirement being the main difference between the Australian and other nations ATPLs
I, and others that had command positions on the DHC-6 with TAA held all of the other ATPL requirements, but because the aircraft was below 12500 lbs we were issued with Senior Coms.
When I moved off shore in 1969 my SCPL was exchanged for an ATPL issued under the U.K. auspices.