It's: Kubota: PLUG, BLIND, Part # 67211-11140 it's on the rear axle case - what does it do? I see a drop of hydraulic oil starting to leak there and it spins. It's like a plastic slotted pan head.
Did you ever find out? I suspect it's just a placeholder for the backhoe mount. At least that's what it looks like from the various diagrams I've seen.
I'm pretty sure if you pop the plug out, you will find a hole tapped for a bolt They are used to keep trash out of holes tapped for mounting points.
My L1501 probably has 20 to 30 on it protecting mounting points on the side of the engine for an optional subframe, and on the back for mounting points for the rops and pto cover.