I have some useful information for you hopefully.
Your movement is a grade 303 movement manufactured in 1925, size 12s, 7 jewels. (The 303 was made from 1903 to 1939.)
It is common to have different movements in different cases, I think the value as a pocket watch comes from the movement, unless the case has intrinsic value as gold or outstanding workmanship.
I don't believe it has a big dollar value, but perhaps more importantly as a family piece it should be possible to maintain it with little trouble in the years to come. I think even ofrei sell these movements.
Nothing to do with this watch per se - but just out of general interest. IWC was founded by an American (International Watch Company). Elgin's name for many years was the "National Watch Company". Some of IWC's most famous pocket watch movements were called 'Elgins'. Don't know why, if someone does I'd be interested.
I'm no expert on pocket watches by any means, but I believe all of the above is sound. Here is a picture of a 303 to compare:
Thanks for showing us your wonderful watch!!