You posted in the right section, there was just a spot of confusion it seems.
If you could post some pictures I may be able to help, but the casting number is of little use to myself nor (I suspect) the friend I have who works for Toyota. I'll talk to my friend and see if there is anything characteristic to any specific year of the R154; I don't know of anything.
Here is the service manual's page:
http://www.cygnusx1.net/Supra/Librar...Section=MT&P=2