Hi Maurice,

The 1956 model looks like it has the 4 blade plate ,the easiest way would be to get a Victa Rotomo blade plate
and convert it to a 4 blade plate.

The other way is to buy a complete mower for the blade plate, but a Collect O Matic would be very rare.

https://www.outdoorking-forum.com.au/forum/ubbthreads.php/topics/60224/collect-o-matic-c1956.html

Cheers
Max.