Hi Terry,
At this time the FAA still show it as a plain old 790 by the way. Apart from 737 300/400 conversions we have standardised to F for pure built freighters and SF for conversions. The 300/400's change to F when converted. For example there are 747 conversions which I have seen with BDSF but we don't use the BD designation on them either as there are conversions centres other than Bedek/BD.
I will need to try and find a copy of the STC to see what the actual designation is given for the variant
Alan