Csharvey,
The 3rd type Crass and the Lewis models were very similar. As Drew noted, the Lewis models are suggested to have started at ser# 55000. Ithaca Gun Co. normally used up existing frames when transitioning to new models. The major change made to the Lewis model was the addition of a top lever bolt through the barrel extension. Can you post a picture of your gun showing the side view of the frame with the barrels open.
Also, the threaded hole in the forend hanger/lug is not factory. The solder joint appears loose in the pictures and the lug has shifted, not allowing the forend to latch properly with the J-spring. Most competent gunsmiths can reposition the lug and resolder without need of a reblue.
John