Category I of the ITAR controls firearms. However, it ONLY controls shotguns that are combat shotguns, along with pistols and rifles to caliber 50.
Category I (d) in part 121 of the ITAR specifically controls "Combat shotguns. This includes any shotgun with a barrel length less than 18 inches." The ITAR does control ALL other firearms other than non combat shoguns. The Dept. of Commerce controls non combat shotguns. (shotguns with barrel lengths of more than 18")
The Export Administration Regulations, or EAR, through the Bureau of Industry and Security specificially controls non combat shotguns. The ECCN for non combat shotguns is 0A984.
0A984 Shotguns with barrel length 18 inches (45.72 cm) or over; receivers; barrels of 18 inches (45.72 cm) or longer but not longer than 24 inches (60.96 cm); complete trigger mechanisms; magazines and magazine extension tubes; complete breech mechanisms; buckshot shotgun shells; except equipment used exclusively to treat or tranquilize animals, and except arms designed solely for signal, flare, or saluting use.
Since a lifter is not one of the parts above in ECCN 0A984 (a part not listed above from a non combat shotgun) it then defautls to EAR99, which is no license required. This lifter WOULD be regulated by the ITAR if it was a part off of a combat shotgun and would fall into Category I (h).
I have a significant amount of experience training and education specifically involving the exports of firearms, Categories I, II, and III of the ITAR, and multiple ECCNs from the Commerce Control List, I would invite you to message me if you would like some additional guidance. Having said that, if you are still uncertain after reviewing these regulations yourself, I would suggest you contact the State Department directly. Another good resource for you would be BIS. I have attached the info below.
US DEPT OF STATE Response Team
http://www.pmddtc.state.gov/response_team/index.html
Bureau of Industry and Security:
http://www.bis.doc.gov/
Ultimately if you export this item, you are solely responsible to ensure you are in compliance. Take a few minutes and look over these websites. Dont hesitate to contact me for more direction if needed. Good luck!
By the way, I would try what Kurt mentioned. Hard to go wrong taking advice from a guy with that much proven experience and success. Not to mention I know he has had a gun or two come across his bench.