Is that a 19-inch? 19" mulch plates are tough to come by.
I more recently bought an 80's 5253 19-inch pusher with the 4.0 F that looks just like that. It only came with a discharge plate. I inquired about a mulch plate, but the guy hesitated and said he didn't have one. I personally think he did, but didn't wanna sell it with the mower. After we talked a while, he showed me a couple older LBs he was working on, so that's why I have a feeling he was harboring a mulch plate for it, lol. At any rate, I'm the mulch-type, so I wanted a plate to work with this one. I asked around and was told that a 21" mulch plate would work on it.. Well those I have, so I took a 21" mulch plate normally sold with mid-90's/early '00 models and installed it with great hopes.. only to find out it still doesn't fit right and work, there's still an opening so it's not fully blocked off. So, defeated I re-installed the discharge plate. Then I came across a couple older 21" LBs on CL that I picked up, one a 4.0 F like these, and the other with a 4.5 F. Both came complete with side discharge plates, mulch plates, and side bagger setup. For the hell of it I decided to try one of the mulch plates from these mowers on the little 19", and it fit! This is the style plate that worked with it, so this may be a style that will work on yours, and again it was from a 21" and fit fine. I think the PN might be 609806. I'd have to look on it to be certain though.