That is very possible because the Speed Six frames are slightly different from the Security Six frames. Several of us have found that the measurements of a Sped Six and a Police Service Six, in the grip area are different by a measureable amount. When I read you post, I measured the top straps of scalloped and non scalloped, blued and stainless and found some measurements to match yours but found the majority were even from front to back. One oddball was .200/.200/.200 (this was a 150- s/n low back fixed sight Security Six). The others (blued and stainless, PSS and SS) were all in the .240/.240/.240 area.
I would theorize that the differences that show up were probably caused by the casting and machining of the frames.
