Could it be that the .mtl function is a hold over from Poser 3? It had no transparency to provide, and maybe this part was never altered in the two subsequent Poser versions. The RDS plug-in was Poser 3 to RDS 5.0. It works in RDS 5.5 and it works with Poser 4 - after a fashion. I think it does the part of P4 that is of P3, but not the additional functions.