M-100 to M-111
These trailers were painted maroon and yellow and I believe that they were leased, as there is an "M" before the number. The M&StL trailers that were painted red and white have an "S" before the number indicating that they were owned by Spellacy Motor Cargo, Inc. The "S" also appeared on GN trailers that were leased instead of the usual "G" that GN used.
This photo of M-108 and M109 was taken by Gordon Lloyd of Peoria, IL, on September 16, 1961 and is in Lou Schmetz's collection. Also of note is that the M&StL was still using jacks and chains rather than the ACF hitches that most roads started to install in the latter part of 1958. There are also signs of other lettering under the yellow paint.
You can use the On Trak 24' trailer or the 24' Athearn, modified.