Need help ID shotgun and whats missing

[wils33]

Can anyone id this single shot shotgun. It is a 12 gauge. Also, what part is missing and broken. The forearm wouldn't stay on. There is the letters EO and a three digit number on the barrel under the forearm and on the inside on the receiver. Also, a I with a circle around it on the right side and the word special on the left.

 

[NailGun]

wils33, First...I am no expert...
This looks to me like an older Harrington Richardson (H & R) shotgun. I can't tell by the pics. what model it may be. Here is a link to check out. It will bring you to a H & R model 580, which may be similar to what you have. Most single shot shotguns share similar design features and have similar parts arrangements. There is a schematic showing what parts should be present (they also show extra parts for other model variations).
Hope this helps a bit. Good Luck!
NailGun