Dear sfphotoart,
I have a Mamiya 6 IVB. The English manual I downloaded from Butkus says your 16 is my 4 and it's the picture number arrow. I don't claim to be 100% correct on that, but it's the only thing in the English manual on the right side of the camera near the top plate.
You might want to download a manual from Butkus.com to compare.
Regards,
Tim Murphy
Harrisburg PA 🙂