Four major changes with this paperback series.
1) Change in numbering from lower case d1099 to upper case D1099
2) Crest Books brand logo changed to Fawcett Crest brand logo that remains in use as of 2011.
3) During this portion of the series, Fawcett stopped listing printing and printing date information.
4) ISBN changed from 232-01099-050 to 449-01099-050.
Known printings:
21st printing, May 1970
There are versions with no printing or printing dates listed.