I'm not crazy about the type of research I am doing, but lacking anything more substantial...
It was pointed out to me on the NH Forum that CDS previously produced Rexall Chemical dry transfers, I was able to find a vendor with the lettering diagram (O scale dry transfer set) posted on line. The dry transfers are for a 2600 cu ft capy Airslide GACX 44373 with the note "1959" visible on the drawing. My January ORER ends with the 43991-44199 series listed.
Now, I never used CDS dry transfers but I know they had a good reputation for a quality product, so for now I am going to "assume" CDS had a photo or other data to indicate there were Airslides leased bu Rexall Chemical.
This means I can now go back to one of my original questions;
Is there any source to know to whom General American leased Airlside covered hoppers?
In the event the Dry-Flo hoppers were built too late in 1959 for my purposes, if there is a way to learn the car numbers for the Airslides leased by Rexall Chemical -it may be possible to confirm if some were leased, say, in 1958 based on the GACX car number alone.