Apologies: I meant a Royal 10. Thanks for catching that. Im in desperate need of one.

Uwe, thanks for helping me nail down the right carriage. I was told it's an 11-inch carriage. If I measure just the platen rubber, it's 10 inches. I will have to look for the serial number. Here's a picture of it: both sides have windows. 

I suspect that it will fit, and that the platen on the later model 10 should be 'useable', but the odds are that it will be rock hard. Had your existing platen been repaired (if it can be) and resurfaced, it would have provided the performance of a like-new, which the replacement platen won't. Also, you'll still have a Royal 10 that needs a platen (it seems a real shame to turn what might be a perfectly good typewriter into a parts machine for something like a platen). 

​Anyway, good luck, and I hope you'll provide us with updates on your platen swapping experience.

I'm glad you two chimed in on this one. After buying the second Royal, I had second thoughts about my logic and felt uneasy about the whole thing. After all, I've never cannibalized a machine. So I will order the new platen and enjoy the second, newer machine while the platen is done. I'll take this a $100 lesson. I'll post pictures of the incoming 10 and a type sample as soon as it arrives.