given the span between '87 and '93 thats a pretty wide span of time.
I'd be willing to bet that even though they look similar there have been changes made even, slight ones that may make all the difference in the world when it comes to identifying the parts and getting the numbers for them.

I would make sure i had the right manual according to the serial number of the truck before trying to determine the breakdown of the valve if it were me. And if this is a japan built truck it could be one of those gray market trucks which can be aggravating to get parts for because of that ;o)