Seems like an topic that's actually really interesting come to think about it. Looking through photos of steam in its glory days I came across Milwaukee Road 265. As you may know she is preserved in Illinois at IRM. However, I came to the shocking conclusion that she had modifications between her build date and display time. Along with a no longer present horn (which was originally on all S3's) is a much larger smokebox door, adjusted headlight, and some higher cylinders than her modern counterpart, and present day 261. I also found that a scrapped version, 264, looks like her preserved sisters. Is there any records that prove that these engines were modified between 1944 and 1953?

Credits goes to Richard Leonard's random steam collection. The links below are of 265 in her early days, and 264 for a comparison. . I suggest you check him out, as well as his fantasy steam concepts

http://www.railarchive.net/randomsteam/milw265.htm
http://www.railarchive.net/randomsteam/milw264.htm

There are many discussions about the 'history' of the S3 smokebox fronts, the offset bell location, etc.

The 'all-weather' smokebox front was apparently an early 'retrofit' and evidently provided significant advantages in maintenance. Note that the offset bell was offset on the original ("USRA") front too. When the Mars light was installed circa 1950 it was logical to put it in the 'blank space' adjacent to the bell. Now it is off again, to give a more 'as built' appearance, which some (myself included) think leaves the smokebox front look as if 'something is missing'

Apparently IRM has an 'original' smokebox front that is either correct or very close, and there have been discussions about installing it (which I was told foundered on issues of price more than anything else).

If I had to point to a modification to be 'undone' it would be the headlight visor, which was fine for an S3 imitating a DL&W locomotive but is just wrong for a Milwaukee engine ... shades of those shiny numberboard frames that reputedly got one fan so irritated at the 765 Berkshire people!

But I'm glad it's in operating shape, whether 'period-correct' or not.