From the angle, it looks like a semi to me.
According to a Google search, the average SQHB is 6.5-6.75 inches wide so this is probably a wider SQHB.
I
think you measured it right, I think you're supposed to do it from concho to concho like you did...
Here's the link I was going off of for most of that^^:
Western Saddle Fitting and Different Tree Sizes