MCI did make the MC-1 Challenger in the '50's but it didn't look anything like the Scenicruiser.
The MC-1 was a 35' coach with a straight roofline.
The Scenicruiser was a 40' deck and half coach with a step roof and windshields in the step.
It wasn't until the MC-6 and MC-7 that MCI coaches had a step in the roof. By the time the MC-9 came to be they were back to a flat roofline.
Several manufacturers made coaches with split decks and roofs with steps.
The most common were made by Flxible followed by Mack, Crown, and several other small coach companies that went out of business in the '60's.
GM had the lock on the 40' coach market until MCI came out with the MC-7 in 1968.
Eagles, made by Kassborher in Germany, started to appear in the late 1950's but were exclusive to the Trailways system for many years.
Which is all to say the bus you mention may be an MCI-1 but it couldn't be a Scenicruiser.
Here is a link to MCI
http://www.mcicoach.com/AboutUs/timeline/timeline.htm