William,
I'll say this again. There is no such thing as an Improved I frame transitional chambered in .38 Special manufactured by Smith & Wesson.
NO I frame of any type, original or improved, was ever chambered in .38 Special.
It is dimensionally impossible.
The J frame was the first sub-K-frame revolver chambered for .38 Spl.
From Supica and Nahaus (The Standard Catalog of Smith & Wesson), page 110:
The J frame was developed as an attempt to package the power of the .38 Special cartridge in a small, pocket-sized revolver. The I frame was too small, although it would accommodate six-shot .32 (Long) or five-shot .38 S&W configurations (the Terrier).
ibid, page 108:
I Frame .38
The .38 Regulation Police and the .38/32 Terrier are five-shot revolvers chambered for the .38 S&W round, which is shorter, older, and less pwoerful than the .38 Special round... The I frame was too small for the .38 Special rond, and was eventually discontinued and replaced by the J frame, which would accomodate both five-shot .38 Special and six-shot .32 (long) configurations. Also note that the averate .38 Special cartridge will not fit within the frame opening of an I frame or "Improved I' frame, which has a frame opening of 1.52" by 1.33". All examined I frames and Improved I frames have a nominal cylinder length of 1.25", thereby requireing the new "J" frame with a larger frame opening to fire the .38 Special cartridge. This new frame is 1.645" by 1.34" with a longer cylinder having a nominal length of 1.4", thereby accommodating the .38 Special.
Also, Smith & Wesson's official historian, Roy Jinks, makes absolutely no mention of I frame .38 Specials in any of his writing.
The Model 11 has nothing to do with this discussion, it's the K-frame follow on to the .380-200 revolves, which were made for the British and Commonwealth governments during WW II.
If you like, I'll even e-mail Rick Nahaus, one of the authors of the Standard Catalog, inveterate collector of S&W revolvers, and close collaborator with Roy Jinks on the materials that went into the two published editions of the Standard Catalog.
I've seen supposed I-frame .38 Specials offered at gun shows before. Not a single one has passed the sniff test. They are all very early J frames. The dimensions don't lie.
I have to know, though, what makes you think that you have a factory Improved I frame chambered in .38 Special?
Have you ever had it authenticated by Roy Jinks at S&W?