The London Armoury Co manufactured both the Enfield Rifle and the Kerr Rifle. The design was such that the barrels were interchangeable. The Volunteer could purchase an LAC 'Enfield' and a .45 cal Kerr barrel. This would enable him to shoot with the Government arm of issue when required, but in open competitions where small-bore rifles were permitted he could swap barrels, thus avoiding the expense of two rifles.
Re. Jac Weller's article, is he not just using the term 'Enfield' in a generic sense (a 'military match' rifle may be more appropriate)? I don't recall that he made any reference to HIS rifle having any Civil War provenance. It could have come from anywhere. I had always read the article that he was shooting a small-bore rifle of an 'Enfield' pattern, just to complete his study. He writes "there was the more conventional .45 calibre Enfield with both Henry and Turner rifling," but never clearly identifies what it is he is using.