I’m pretty sure Joe Breeze used Sub 11.0 dropouts on some of his SS mountain bikes...
A little research turns up info that is easily found all over the interwebz that Sub 11.0 was a QBP company, as is Surly, and that their dropouts were used on the earliest 1x1s (like in taras0000’s link) before they started labeling them as Surly, so if you are sure your bike not a Surly, its very possible that it is another QBP brand.
Good luck...