For inputs, you need switched. There is a little tab that touches the tip connector when nothing is pluged into the jack. In amps, this gets connected to ground.
As to why some Marshalls have metal nuts and others plastic, my guess would again be cost. As the years went by, Cliff brought out new products which were less expensive, and Marshall being a normal manufacturer bought the cheaper model. That's just a guess, mind you, but I'm probably right. The main body of all Cliff jacks is plastic (one of the reasons I'm not wild about them), so there isn't really all that much difference. If you can, I'd get Switchcraft jacks and some isolation washers. You'll have to enlarge the jack holes a tad, but it's pretty easy to do if you have a step bit.
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