Ken, it's good to see you're still active in the civil war artillery field. Your search will be complicated by the fact that the Confederates captured more than just a couple of yankee-made James 3.80" Rifles and Rifled 6-Pounder (3.67"-caliber) Smoothbores which had 15-groove rifling. For example, the Dickey-&-George book shows a fired 15-groove 3.8" Read Long-sabot bolt dug at Charleston SC, and also a 15-groove 3.8" Read percussion shell dug at Chickamauga GA.
You say the 3.67"/3.8" Rifle you are researching was made at Mobile AL. So, perhaps the following is most relevant to your search. Long ago, I owned a fired 3.8" Read Bolt with superb sharp 15-groove rifling, said to be from either Spanish Fort or Blakely AL. Despite being fired in 1865, that 15-groove rifling was so sharp, the gun was either nearly brand-new, or if it was made years previously, it had done very little firing (perhaps kept in the Artillery Reserve or at a quiet backwater fort).
Steve Phillips probably knows whether those projectiles came from Blakely, or from Spanish Fort.
Also long ago, I talked to a Mobile-area digger who knew exactly where the 3.8" Rifle was emplaced in the Confederate lines, meaning, the battery number. But I cannot now remember the details. Hopefully somebody else here will know.
Regards,
Pete