I did a bit more research and found a couple of images online of the exact frame configuration of my SAA. They are indeed the frames that JP Sauer & Sohn produced for the Sheriff's model (the attached photo is NOT my SAA, but one I found online of a SAA from JP Sauer imported by Hawes--the frame is identical to mine). You can see where the frame was never designed for an ejector rod or tube. Ones that are designed to house the tube look completely different on the tube-side, because the rod has to enter the chamber through the frame. Anyway, from what I gather the frame was intended for a Sheriff's model but was somehow married to a 6" barrel instead (which is still strange because the barrel was never fit with an ejector tube screw hole). Because the serial numbers on the frame and barrel match, I can't see how this would be done aftermarket. Not sure why it was done this way, whether it was intentional or an oversight, or whether there are any others like it produced. So far I haven't found any. I emailed Sig Sauer with a description and the serial number in hopes of finding out, but who knows if they'll be able to tell me anything. Thanks all.