Seemingly there were two (B3401 and B3403) and had petrol engines.
In 1936/7 they were re-engined with Junkers 90 diesels (become D3401/3403).
Their design was heavily influenced by similar buses operating in Detroit and were used on interurban tourist routes from Rome to Tivoli (along the long and straight Via Tiburtini) and to Ostia.
Most of the photos I have found so far are on Pinterest which is a pain to access unless you have "joined".
Citroen were, at this time, famous for design flair, but maybe this was a flair too far?
This link doesn’t add much but may be of interest.
There are also some layout plans- obviously this is where your Crellin came from.
At one point in the film it looked as though the retractable steps for access to the upper seats blocked the door on the lower level.
This page is exactly for what the heading states, it does not matter if you do not have a picture of your particular ‘Ugly Bus’ somebody may be kind enough to contribute one for us all to view.
31/01/17 A Red and Silver Ugly Bus Again, no idea of copyright – what is this then?
Over to the experts Roger Ingle Roger, this was, I believe, a weird Citroen U55 Cirrus, built for Paris Cityrama tours.
I photo’d it (or one of them if there were more than one) in 1961, and it certainly turned heads.