The seat cushion top is 15" from the floor level.
I have one box seat in my M109.
Many years ago, I bought an M185 from Ft Sam with that box seat in it. Halfway through the 110 mile drive home, I was in so much back pain from that %$#^ seat I was ready to abandon the truck on the side of the highway. But I found that by slouching off to one side like I was driving a lowrider, I could just see over the hood and it didn't hurt as bad, so I got the truck home.
A friend of mine wanted a box seat, because with the back removed, it would allow him to fit in his M109 (he's a big boy!), so we traded my box seat for his springer. Both happy.
Some years later, he traded the M109 to my son for a CJ-2 jeep. After a couple of years, my son decided that since he rarely had time to drive or maintain the truck because of work/family, he was going to sell it rather than have it sit and decay.
So I bought it from him, and that wretched box seat came back to me! But I had picked up a spare springer seat in trade from Quonsethut when he built his gun truck and put a box seat in it, so that one is going into the M109.
Cheers