While researching for the CGW Color Guide I was handed a list of covered hopper assignments. The CGW covered hoppers are of the same type and same age as those in Bill Michaels original question.
While the cars in a given series were identical, they were assigned to a specific service AND a specific loading point by car number. The cars would have been loaded at the same point repeatedly and unloaded wherever they were sent. Weathering would eventually become the only clue to the assigned service if there was any clue at all. For those cars painted black and assigned to cement service the weathering pattern should be obvious.
These short, 70-ton capacity, 2-bay covered hoppers were assigned to service hauling the aforementioned cement, fertilizer, lime and meal. The lime weathering might resemble the cement weathering.
The M&StL received its first (and only) grain hoppers in 1958 from Pullman-Standard. Imagine taking your Kato car and stretching it to a 3-bay configuration and you have the M&StL's grain hoppers. These were certainly very small cars by today's standards. The M&StL was rarely an innovator so I imagine some other railroads also had these grain hoppers before the M&StL
In the M&StL's case, cement hoppers were stenciled for cement service and the grain hoppers for grain service but this was for advertising, not to instruct crews doing the switching.