There may still be a stamping somewhere on the props. I would try looking closer or in different areas. Between the blades, around the hub, etc. You might need to take them off to find it. You'd still need to know what you have to compare it to others.
Regardless of that, how does the boat run? You might be asking questions and changing props for no reason. With a minimal load in the boat, can you get the engines into their max RPM range (preferably the upper end with a light load)? Run the boat WOT, trimmed as high as you can before loss of speed or ventilation.
