I did a quick check and it's also available to buy on East Coast.
One thing I have noticed is that this particular product (FCR - Off Peak 1st Day Return) seem to be missing/not present from all SWT set flows I looked at.
The only ones which I can see in avantix as a whole are (FRX - Off Peak 1st Day Returns) which have peak restrictions,whilst Southern set FCR's that do appear are valid on any train after 10am.
The only possible conclusion I can come to is that these FCR coded tickets are either fairly new (on SWT flows),have errorsly been ommited from Avantix or are only available to buy online. However in all honesty I have no idea for sure.
Cue someone who does know to tell us. **Cough**