This is the pre-cursor to a very long series or Boris Vallejo calendars. The famous Tarzan Calendar published in 1978 by Bantam came in a mailable cardboard carton.
Boris Vallejo
Index: 01-January
Index: 02-February
Index: 03-March
Index: 04-April
Index: 05-May
Index: 06-June
Index: 07-July
Index: 08-August
Index: 09-September
Index: 10-October
Index: 11-November
Index: 12-December
Index: 13-Centerfold
Unset
Index: 14-Back-Cover-Cover