Those owners with affected serial # L and MX series should be getting a letter from Kubota. I got mine for a new L4060 HSTC on Saturday. I talked to my dealer and it requires splitting the tranny and applying Loctite to a bolt if it's loose.
If you havent got a letter, your dealer has the list of affected serial numbers and can check yours.
The problem is that when the dealer goes to put the tranny back together there are 2 gaskets needed. The gaskets for the MX series are plentiful, not so for the GL series.
This is the part number of the HST gasket which is in short supply TD350-58190, the other is for the HST Mid-case TD270-50290, I'm told there these gaskets are available.
This is a picture of a L3560 which was apart for the inspection, notice the locktite, and also the MX torn apart for inspection in the background. Both tractors were dealer tractors on the lot. Mine was the first customer tractor in the shop for inspection.
This is going to impact alot of tractors, if you have one you better get in line.