Well now I have discovered that the 1977 edition of the Stoeger Shooter's Bible lists the Iver Johnson Cadet in the following calibers.
6-shot, .22 Long Rifle
6-shot, .22 Magnum
5-shot, .32 .38 S&W
$52.00
5-shot, .38 Special
$59.75
So apparently they did make a .38 Special, but no mention of adjustable sights, or a ribbed barrel.
This time period of Iver Johnson history has not been actively researched, and so far as I know the conpany records are no longer available. Therefore I must speculate.
It would seem that they might have had a surplus of frames, which had an adjustable rear sights. At the same time there was a demand for .38 Special snubbies. So they made some .38 Special/adjustable sighted snubbies, using parts already in inventory - or mostly so.
Numrich Gunparts Corp, that bought out Iver Johnson's remaining parts when that company were out of business, does not list any .38 Special cylinders - so far as I can find.
It is probably a rare breed, but only of interest to a limited number of Iver Johnson collectors - but they might be very interested.