It could also just be that he got it through more mundane means (found it? Inherited it from his teacher? Summoned it from some other place?)

Saber's pretty matter-of-fact about him possessing the scabbard, and it seems to me like her being summoned at all was contingent on the scabbard as a bond to contract with her (but that may just have been the condition for Shiro, because of his lack of training). Still, if he had it before he had her, it would make sense for him to be able to use it to summon her, and would also make sense for him to have it afterwards.

We don't really know a whole lot of anything about the elder Emiya, but we know he was a particularly competent magician, who was also apparently pretty handy with a gun, and had a certain degree of morality beyond the expectations of the average magician... who knows? Maybe he's Merlin's heir?