Is that 4.5" exactly, or just under 4.5"?
Until then, we have fresh information to use. If this is a Confederate APDS emergency production round for use against Yankee gunboats/tinclads, then it may have been cast as close to the bore diameter of the intended gun tubes &
without bourrelet rings.
30 lb Siege Gun is a 4.5", designed to lob 30 lb shells. Someone cranking out home-brewed APDS bolts might have been casting them real close, particularly if you add in the weight of the missing sabot and the stud.
http://civilwarwiki.net/wiki/4.5_inch_Siege_Rifle