Give them credit Victa replied pretty quickly (a few hours) stated they use the GT-MC600 gearbox in the Mulchmaster 560, and they've used the same gearbox in it since 2016.
https://generaltransmissions.com/en/products/mc-600-602/

Pretty ordinary gearbox, I must have mixed Victa up with the Maspot Contractor:
https://masport.com.au/outdoor-garden-products/petrol-lawnmowers/contractor-/contractor-st-s21-3-n1-sp-k

I think it was masport who released and had to upgrade the Gearbox after issues. Seems they've used a superior gearbox to Victa, stating they use a 3 speed pro range gearbox from GT - Victa's one is from the residential range.
https://generaltransmissions.com/en/product_category/gas-commercial/

Both are less robust than Deutscher but thats a very high standard.