I sold a set a of 69/70 pedals on ebay, here's a photo of the rubber bumpers. Unfortunately without crawling under the dash of my Parisienne I don't know which is clutch and which is brake. As you can see, one is definately thicker than the other. You should be able to verify brake bumper thickness in your Laurentian.
Thanks! Looks like I'm missing the fat one (brake) in both cars, only have the thin (clutch). The ones on eBay -might- work for me but the right part number, according to the Impala assembly manual sheet 5-A2, is 3723230.