Have the barrels looked at and gauged.

I responded to your post on the original thread with some low pressure links, but those shells do not, in any way, guarantee that the barrels are safe to shoot.

I just got my first Ithaca double. The serial number "671XX" shows it to be a 1902 Lewis 12 gauge grade 1. It won't lock up. I see a pin blocking the locking bolt from engaging the locking lugs on the barrel. I'm a pretty good gunsmith when I've got a diagram but I can't find anything on this gun.

I can send you diagrams when I get back home to my laptop...

Bach, have you had a chance to find the diagram for it yet?

Does the top-lever stay open when all the way to the right, or does it come back to center on it's own?

I found the problem why it wouldn't lock. The lever stays open to the right when open. But when I close the action, the lever doesn't have spring tention to keep the lever centered. It has enough mention to lock it closed, just not enough to keep the lever centered.

The locking mechanism is quite adjustable to solve that problem. It may just be gunked up and in need of proper cleaning. Assuming that there are no substantial cracks in the head of the buttstock, a quick test would be to open the shotgun fully assembled, make sure that the top-lever is locked to the right on it's own so that the barrel extension doesn't hit anything, and snap the gun shut with a bit of force (open palms should be sufficient). If this doesn't make a difference, then you're probably in need of a bit of adjustment due to age and wear.

It turns out I need a new locking bolt. Someone in the past drill a hole in it so the set screw wouldn't let the straddle block from slipping. So I can't adjust it now.
Anyone know where I can get a new barrel locking bolt?

You may be able to fill he hole (drill, tap, plug screw, and file). But, it's probably easier to buy a replacement.

Here's a link to the part listing. Sometimes the parts that are listed with Numrich are mis-titled, so it can be a bit of a crapshoot. Also, I'm unsure if the bolt for the Crass model is longer than that on a Lewis. Of course, you could cut and file the longer one to the proper length for your shotgun. Anyway, try this one: http://www.gunpartscorp.com/Products/1007820.htm

Thanks. I've been considering all of the options you mentioned. I'm thinking about asking Numrich for a picture of that exact part.