Opel 4spd box was a piddly little thing introduced for HR and was also standard on HK GTS and HT-HG 6cyl GTS. It was made in the Phillipines I think. Option code was M20.
The Saginaw boxes were standard on GTS327 and optional after Monaro release behind V8 in most HK-HG models other than Brougham. The V8 box was code M21. The M22 Saginaw was also available as the 4spd option behind 6cyl HK-HT commercials, and was a heavy duty 4spd option on some other 6cyl HK-HT. The M21 was also optional behind some 6cyl. There was a 3spd Saginaw used in LHD LH Torana too. Saginaw was a USA made transmission. It was the light duty manual used behind smaller engines and lower power/torque V8 engines. There was a slightly heavier duty version used behind the 275hp 327 in Camaro and Nova, which is what we got here in HT-HG GTS350. Anything bigger in the USA used a Muncie like was fitted to HQ GTS350.

Holden had a problem with the Opel 4sp box, because it was assembled in a shed with Chooks, so feathers and shit got into the box's, that's just how it was back in the day in Asia, the Chooks have to go somewhere at night. so I was informed.
Other than that it was a good box and just as strong as a old Holden 3sp non syn 1st type in the EH-HD-HR I believe, it was not as strong as the latter Aussie 3 and 4sp we all know and loved.