what will the frequency be if I cut an Apollo Matchflex-R shaft for a 5-iron to 37 1/2″ and use a standard Mizuno MP-59 iron head?
Really can not tell you. Too many variables. If I had a Mizuno MP59 iron head, I suppose I could mock up one to tell you what the frequency would be, based on a 5 inch clamping dimension. Unfortunately I do not have the head. It is important to have the head when the request is as specific as yours because the head weight and the hosel depth play a role in what the frequency will be.
Britt Lindsey