Over at the Woodnet forum, there was a request for plans for a spring pole lathe. A few people chimed in with photos and links to various sources. I dug up a reprint of Roubo and scanned out the two pages for the forum in jpeg format. While I'm at it, here they are:
and for good measure, here is a higher res version in PDF format.
I have some original Diderot plates but they are no where where near as precise as the Roubo plates are.
Till next, Gary