Sentinel was one of about a dozen early Mead Cycle Company brands. They used a lot of head badges. Your bike was manufactured either 1897 or 1898. The American Wheelman site has it listed for only those two years. It takes 1/2 inch pedals. It looks like a collet seat post binder. If it is, your collet adjusting bolt is missing. You can’t get the seat post to stay adjusted on a collet post binder without that large bolt. If it’s a two piece crank then it will be extremely difficult to get it apart for grease and adjustment. More photos of the other side and closeups of the crank arm and seat post area will help. Nice bicycle. It needs to be ridden again for sure. A lot of these old American bikes had rear hub spacing of 120 mm or close to it. Check the rear spacing before ordering wheels. The axles probably won’t fit the rear drops so those will have to be filed wider. If you use a vintage front hub that will probably fit the holes in the front fork, modern axles are too big. Modern front hubs are also too wide to fit the fork so vintage 96 mm wide hubs are what you need. For the front fork, since there are just holes and the wheel won’t drop out, adjust the front hub axle off center, put the long side in first and reef like crazy on the front fork to get it to spread enough so the short part of the offset can be forced in.