Re: OK quick prop question???
The entire "48 number" is the Merc part number. The 19P is added for your benefit, so you make sure you are getting the right pitch. You cannot buy a different pitch prop with the same "48 number".
Trying to break it down is silly, since Merc may or may not have simply assigned the next allocated PN to it. The A4 likely refers to an update to this prop(from A3) that changed the part to add cupping or different paint or different type of hub, etc. Without detailed info from merc, you simply do not know why the part number was changed.