Stevetype33 wrote:

how interchangable are their parts (if at all)?

That's an excellent question because it leads to the bigger picture of parts sourcing. 

I have little experience with Underwoods, so unfortunately I can't answer your specific question; however, when I first started poking around the insides of the typewriters I was buying I was struck with a strong feeling of déjà vu. I was recognising components and individual parts that appeared to be identical between the various models produced by the same company.

Of course it makes sense that a typewriter company, or any other manufacturer, wouldn't necessarily reinvent the wheel with every new model release, so I really shouldn't have been surprised by my discovery. 

I'd be surprised if there weren't some - if not a lot - of parts that were carried over from one Underwood model to the next. Perhaps the ultimate resource for a typewriter collector would be a parts compatibility chart for each brand and model. It's a pie in the sky idea, but could you imagine being able to look up a specific part that you need and then know that part could come out of any number of different donor machines?

Is this what you're trying to do, replace a part from one Underwood model by hopefully finding it on another?
 

Stevetype33 wrote:

The third choice is to try and source some rubber hose of the right size and re-cover the platen myself.

I've read that heat-shrink tubing is also an option. The rollers in an Underwood that I recently bought are completely shot and consequently it won't grab a sheet of paper, so I've been thinking about trying out this method.

Please keep us posted, especially if you try recovering it yourself as I'm really curious as to how well that works. I've been very lucky so far to not need a platen, but I'm sure the day will eventually come.

A number of Underwood four-bank machine variations were made, but as for Underwood three banks, I think there was only one basic design.