Amtrol indirect fired model WH-? / ser.# BM67281.
That is the same sequence used by Bradford-White (who makes most of the WHs for other brand names).
The first letter is the **year **of manufacture…B=1985 or 2005
The second letter is the month…M=Dec.
Or it could be something else…depending on who made the unit. I have no informay tion on Amtrol.