FYI, in the U.S., prior to 1968, no federal law required serial numbers on firearms, and most of the less expensive guns didn't have them.
I can't find Union on my lists of "tradename" shotguns, but there was a Union Arms Co., of Toledo, OH, which made shotguns c. 1903-1913; maybe someone else can provide more information. In the 1880-1920 period, many gun makers, here and overseas (mainly Belgium), turned out inexpensive shotguns literally by the ton. Most were serviceable, but few were of anything approaching high quality. Hundreds of names were used - Cresent alone used over 100 trade names on orders from hardware stores, distributors, etc. At one time, anyone ordering as few as 10 guns could have any name he wanted put on them.
I think is a safe bet that parts will not be available, so I am afraid you have a wall hanger at best. I would suggest you not spend any time or money attempting to "restore" or repair that old gun.