Does anyone happen to know if a scan of Mazur's report exists, and, if so, where to find it? It appears in the references for Katz's "Higher congruences" and "Eisenstein measure" papers.
I have it. Mazur gave me a xerox copy off his shelf when I asked him (in grad school) if a copy exists. It's 56 pages and the first sentence is: L'objet de ce rapport est de construire la série L p-adique de Kubota-Leopold et d'établir quelques propriétés fondamentales. It was in my office and I was going to try scanning it this evening, but literally as soon as I stepped into the elevator to go to the scanning machine a fire alarm went off in the building. So I had to leave. On my way out someone said he saw smoke in the hallway of the physics wing of the building. But even if the whole place burns down, fear not! I took Mazur's paper with me. You'll get a scan in the next few days.
Update 1: A scan has now been made, and my department's building did not burn down.
Update 2: I sent the scan to Mazur, who has posted it on his homepage at the "Older Material" link, so everyone can get a copy for themselves. A direct link to the scan on his homepage is here.