As I have mentioned before, I do not collect rookie cards per se and will never afford a m101-4/5 Ruth, so I don't have a lot invested in the debate as to whether one is the rookie and the other is not. I agree with Jay that there does not appear to be any hard and fast rule.
As I have thought about it more and done more research, I now would argue that either can be considered his rookie. Previously I believed that because m101-5 was printed first, it must be the rookie, and the best way to make sure is to acquire a card that came only associated with that set. Now it seems more logical to accept either of them as rookies, at least to me.
The cards were printed only 6-7 weeks apart max, at least first-run printings. To me the question then begs what would make one the rookie--when it was first printed or first available? Leaving aside the blank-backs for a minute, the answer is pretty murky.
Mendelsohn offered m101-4 in the Sporting News on April 6, 1916. In theory then, one could have ordered and received the m101-4 set in mid April or so, including the Ruth. Successful Farming-- one of the three sets exclusive to the earlier-printed m101-5, was not advertised by its publisher for sale until the May, 1916 issue, so it is likely those who answered that ad would have acquired their Ruth's after the enterprising and eager responder to Mendelsohn's m101-4 promo.
Similarly, Famous and Barr, another m101-5 Ruth producer, started its advertising April 8th, 1916, technically after the m101-4 cards were already available. More importantly, the store released the cards in weekly series of twenty cards, alphabetically, and Ruth would not have been issued to customers until the eighth week--about two months after m101-4 was in circulation.
Holmes to Homes is the only other advertiser of exclusively m101-5 Ruths. Its advertising also began about a week after Mendelsohn's TSN offer. Those cards were available one at a time in loaves of bread, so I suppose someone could have pulled a Ruth the first day or so, but even then, it is at least theoretically possible someone, say a kid in Chicago, already went downtown and paid at Mendelsohn's Peoples Gas Building to receive his set of m101-4. Under all of these circumstances, how should it be determined which came first for rookie purposes?
As for the blanks, we do know that m101-5 was printed first, and presumably would have been available before m101-4. However, since the Ruth card is identical in both sets, you are left with only subtle toning of the card's stock as the sole test of telling whether the card came from m101-5. I doubt the hobby is willing to put much stock in that (pun intended) as a definitive marker, although it would matter to me.
I guess the only way to be sure is to acquire the Chicago Examiner full sheet of M101-5, which issued in March, 1916. Good luck with that.
