The serial number is a six-digit one: 6826xx.
OK, that makes sense now. What you've got is a .38 Military & Police Model of 1905, 4th Change, manufactured between 1915 and 1942, in the serial number range of 241704 to 700000. So, it's highly unlikely (being near the end of the range) that it's old enough to have been used in WWI.
I'd noticed what appeared to be a filled-in hole for a lanyard ring in your photo and thought briefly about whether there might have been part of the serial number in that location, but it honestly never occurred to me that someone would drill out part of the number - obviously after market, as you say. I don't know what the legal status of the gun might be - if the only serial number were on the butt, as is the case for many guns, it would definitely be a rather serious federal crime to possess it. In your case, where the number also appears un-defaced in other locations, I'm not sure.