Try the 4H, but check your lofts.  I got fit for the AP1s and 915Hs and Titleist adjusted the AP1s 1* weaker to account for the hybrids lofts.

I'm a mid-handicapper like you and we need all the help we can get.

Good luck.

Luck was on my side... less than a week after posting the initial question, The 915D2 head cracked on the sole/neck area near the hosel assembly.  The club was covered under warranty and Titleist switched out the red for the blue shafts at no additional…

Hi Mark,
Yes they are indeed completely interchangeable. 

However,
816 H1 and H2 have a 1 degree SureFit Tour Hosel degree range.
915 H and Hd have a 0.75 degree SureFit Tour Hosel degree range. 
This means that if you take your 915 H shaft and put it into…