Hoye has a paper gasket to fit under the triangle plate but I think many of us just slop a thin band of RTV sealer on that plate before re-installing it.
That O-ring inside separates the two sides of the filter, clean vs not filtered, but doesn't have anything to do with an external leak.
That reminds me of what I found on my 'deferred maintenance' :laughing: YM240: The guy apparently used what he had instead of the proper o-ring and it was too fat. It had partly crushed the shoulder on the 'strainer' (filter). At least it was sealed. And maybe it wasn't temperature resistant. The strainer was solidly cemented in place by that deformed, crushed O-ring so solidly that I had to destroy the filter, pull it out in pieces with Vise-Grip pliers leveraged with a crowbar, to get it out. $50+ for a new one.
There were several similar issues to resolve before I put the YM240 service but its been troublefree ever since - since 2003. Well aside from stuff I should count as my own user abuse such as tearing off the oil pressure sending wire while rasseling a downed tree. And an occasional loader hose, likewise.