I don't concur that most pieces are interchangeable in square ruled frames.
"Most" and "...some are" does leave us a bit nebulous in understanding, and perhaps I was not clear enough in my observations about this matter of interchangeability...
My observations are not geographically limited to just one region. I am not suggesting that there may not be regional characteristics that are unique. My offered information covers perhaps most (if not all) the states, from Minnesota to Louisiana and East from there that have surviving Timber Frames and the cultures that built them.
Having now been in and/or restoring perhaps 500 (perhaps more) barn and other timber frames...in an on again off again career of timber framing and historic restoration, as well as working with colleagues that probably pushes this number of timber frames into the thousands...I would have to suggest from direct observation and collaboration...that the frames designed and built employing Square Rule as its primary layout system all reflected a clear and distinct homogenization of parts...which clearly by plan and literary citation was the intent of the Square Rule modality of design, layout and construction.
In some regions all the braces, wall purlins, rafters, and even some Tie Beam, Queen Post, Struts indicated a clear and complete uniformity of design and layout. Some of these members even indicated being mass produced by a local mills with only perhaps Post, Rafter Plates and Tie Beams being cut by the Timberwright themselves. The later you get (1840 onward) the more interchangeability we seem to find, with some regions presenting with such an indistinguishable nature that barnes could be deconstructed and partnered with neighbouring structures with little ill effect or alteration to parts.
In relocated frames, the number of mismarked and unquardentated members is very common. It is only from the passage of time and wear to the frame (and restoration ethics) that keeping members in the same order is necessary on many (not all) Square Ruled vintage cut timber frames. Not only do you find End Bents in the center of structures with new joints cut to accommodate this poor historic relocation at some point in the history of the structure, but braces and wall purlins too in different order and location on many such frame.
Bay orientation has no bearing, per se, on the interchangeability of parts since a span distance is not indicative of interchangeability of joint configuration, yet a simple matter of tangible distance accommodation. Indeed, with large Barns where there may be several bay span configurations of a similar (yet multiple nature) you can and will find parts swapped out on moved frames and can (not that it is standard restoration practice) move members very often to new locations as long as bay span distance is the same.
On contemporary frames...with Line Rule...and even organic shapes...we can (and have) made parts fully swappable...as long as bay span is the same. This is neither difficult nor a challenge (over all) for this method of layout. Sean is accomplished enough with this method to speak to this ability from his own perspective of the Line Layout system's nature of accuracy.
So today, having interchangeable parts, for more simplistically designed Stock Frames (as we call them) is very achievable...even with organic live edge members, which is one of the marvels and wonderful elements to be acquired with adopting and learning the Eastern methods of layout and frame orientation.