J Class Changes

[NW Mailing List]

Sorry, I left my name off the first time.
Can anyone explain the changes made to the J's over their life time.  I'm a little confused in regard to "as built" vs later "in service" examples and what that really means regarding changes over time.  I'm more interested in outward changes, such as driving rods, that could be seen rather than internal modifications.  Thanks to anyone that can give me some insight!
Charlie
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