LetFly,
I have a fair accumulation of Ithaca examples. I have observed that as Ithaca made model changes they used existing model frames and incorporated the new model lockwork. In respect to the bolsters: They can be found on 3rd style Crass, almost all Lewis models, some Minier models and a scant few Flues. The lockwork is what defines the models, not the frames. I am still searching for a transition Crass/Lewis model. I have examples of Lewis framed Minier, and Minier frame Flues. There is also an odd example transition Minier/Flues with 4 pins.