Don't be at all surprised if there isn't one as my present Plus 8 doesn't and nor does it have a plate for filling the R380 gearbox. Rather than cutting another hole and plating it up I left it and top up with an oil syringe from below as you are already there.
p.s. Morgan propensity to leak everywhere when possible made my mind up.
Having said that my previous Plus 8 had a plate for the gearbox only and didn't leak and it was a fairly crude arrangement.
