Supaswift actually did use this blade plate type; it's identical to the one on my 1985ish Big Bob, which I know to be factory original.
As it happens to be off the mower ATM, here's a pic:
Blade and bolt sets are available from the ODK Online Shop;
https://www.outdoorking-forum.com.au/lawn-mo...-swing-back-blade-bolt-set-900174-900173Edit: And UTA is owned by GA Spares; it wouldn't amaze me if they were SupaSwift's OEM supplier.
But wait and see if any other parts are needed, before placing an order - postage is the killer.
Your engine is an 11 cu in, made on 20 July 1983. So it has the Magnetron electronic ignition, rather than contact points. One less thing to have to service!