Exactly what Norm said, though technically in the early days of that style they used a different type of tension washer (the coloured one) which fits on top of the blade and in the odd shape
I thought the different hole may have been a way of quickly identifying a mulcher blade that has the sharpened edge on the opposite side to the normal blades but it looks like a design to stop the blade bolt seizing in the blade.
I definitely wouldn't mix and match the different brands even if they're compatible. Any slight weight differences will magnify as the blades spin and upset the all important balance of the engine.
Ahh, if only victa had kept producing the thumblatch catcher series, they would be in better shape today!
The Victa wider blades are used for mulching blades but I thought the wider blades had an advantage to stay more straight when rotating at high speed ,Rover went to this design too but Rover had them with high lift ,high lift blades aren't generally mulching blades.
Victa uses the blade plate to push the downward flow of grass to the blades for mulching.
