Pete, I tend to side with Chris on this one. I'm thinking it is a fairly modern construct and not a period piece. I think the only other time I've seen it is in this thread (at post 15):
https://net54baseball.com/showthread.php?t=198986
My go-to-guide on Mantle items is the Kelly Eisenhauer series in SCD. It's not listed or mentioned in his Holiday Inn article. Granted, I'm sure there are Mantle items that have not been documented to date, but I doubt this is one of them.
Things that make me think it is not a period piece:
(1) I would think that there would be many more of those floating around if it were actually from the Mantle Holiday Inn. Considering the number of other odd ball sized and shaped HI items that have survived, why aren't there more of these? Kids would have been more likely to save a normal card-sized item like this than other HI items.
(2) I tend to not like the phone number. It's surprising how many items do not include a phone number, especially when you think there would be one. For example, brochures. I have three different Mantle HI brochures, none of which give the phone number for the hotel. Why does this "card" have a phone number? In addition, the HI phone number when it is given on items, is either "MA 4-7400 (area code 417)" [as shown on two different letterheads] or "MAyfair 4-7400" [shown on two different letterheads and various matchbook covers]. The phone number on the "card" is "MA 4-7400." Doesn't match either known version. I know it's a minor point, but why is this presented differently?