Thank you for the replies. The S&S specs get me the right opening hight (I think) - by subtracting the flap opening width from the rib lock distance, further subtracting the rib lock width and the 4 millimeter and something width from the edge of the opening to the rib lock chamfer. With the width - I'm supposing that the film width, plus a bit, (I'm thinking of two millimeters) subtracted from the overall width (in this case 12 15/16") gives the stile width (divided by 2). I'm just wondering if I should make it an even 11" on the assumption that the film width is always less than the nominal size. Like with my 8x10 Lisco the septum width is exactly 8". Have people had problems with film being oversized in width, that is actually over 8" or 11" and so on? This is what I'm wondering.
About the ANSI hight and width dimensions referring to the tray area, or septum area, yes, I wondered about that to. But again the numbers don't work out. The hight is good, just a little bit over. But the width specs are just a little bit under; you wouldn't be able to feed film into a slot as tight as this. But all the other numbers on the ANSI chart look about right. So I'm still scratching my head with this one. Anyway, thanks again for the responses.
Just contact Sandy King at S&S Holders directly. Sandy is a great guy and I would bet that he would be willing to assist you with your questions.