Re: age on an Iver Johnson .38
Definitely not an antique: It's a special order third model, the special order item being the one piece "western walnut" grips.
Unfortunately, there are no special order records to date it to a specific year, but made no earlier than 1909.
The barrel assembly is probably original and not a mismatch, and with a number that large was made either 1916 or 1933.
There are two like yours shown in the book, and both have 4 digit serials with no letter.