That is a mixed up set of pictures. The larger one (or second from left of smaller pics) is the belly side of a Howatt Super Diablo which is a 60" bow. The other pics are of an early Hi Speed which would likely be 58", although they did make some shorter. If it is in fact a 58" bow then you would want a B-50 or B-55 dacron 54" string and Blackhawk is correct on brace height. They were, and still are, great little bows. Good choice, but if it's a Hi Speed you will not see that white glass.
If it does have the white glass it's a 60" Super Diablo which would use a 56" string. Another GREAT Howatt bow.