New L 3800 tool box fell off

If you don't receive a knowledgeable response (this one isn't) you may have to estimate or measure diameter and get a selection of metric pitch bolts to find one that works correctly. Almost certainly metric, not U. S. National Coarse or Fine. Probably be one of the metric 'coarse' threads.

Please post back your experiences so we may all learn.

Oops, forgot to mention. Clean the threads well on both 'nut' and proposed replacement bolt. Don't use a wrench. Turn it in by hand to test fit and test 'jiggle' for loose threads. If you have to use a wrench, it's an incorrect thread.

Lots of times in cases like this, I break down and order from dealer to be certain of fit. Many times the part number in the parts book (online at Kubota.Com) is something a stocking dealer will have on hand. I've found dealer bolts are about twice cost of what Tractor Supply might be asking.