I believe, technically, they would be I and II. But I see them in an Enumeration of Male Voters record (1891, Tennessee) with Sr. and Jr. after their names (I believe it is in relation to each other). All info I have points to them being grandfather and grandson.
Since this is not a ''legal'' document. My thinking was, it could be a casual reference to distinguish them from each other for the enumerator. Or the enumerator used the Sr. and Jr. instead of the I and II in error.
Also, this record is the only document I have seen with any suffix used at all after these two individuals (no record of middle name for either person). And no other documentation indicates they may be father and son. The apparent father of the Jr. is also listed on the record with no suffix (as expected).
Needless to say, I'd rather conclude that the suffixes were used in error, than conclude that I have to start over. But I want to be as certain as possible about their relationships!
P.S. Still have docs to attach.