40 - horsepower
E - electric start (from factory)
L - long shaft (for 20" transom)
O - oil injected - but this was an "auto-blend" system (hope it isn't there any more - battery operated oil tank/pump located between the fuel tank and the motor (I am only aware of 1 (one) that is still working and in use (on a 1990, 90 horse Force outboard) - dozens have failed and fried motors)
6E9 - indicates it was built by Yamaha and was mechanically identical to one of their own branded models - in this case a model 40FM/40FE/40FEML built and sold by Yamaha between 1981 and 1983.
L - reiterates it's a long shaft
424734 - the actual serial number = produced in 1986 for Mercury to be sold under their Mariner line as a model "40C".
