454 Merc s/n's

[TJM]

I have a 1989, 36' Commodore with twin 454 Mercruiser with the Vee Drives. I purchased the boat in 2000. I can not find the engine s/n's anywhere. They are rubbed off of the flame arrestor cover and the rivoted tags by the starter are missing. I have a metal tag by the starter from Mercury Marine that has a number stamped on it, "C333842 R". Mechanic says that is not the serial number. He says it should be an "ODxxxxxx" type format. Is there anyway I can cross-reference this or year of the boat??