Lost one somehow. Musn't have been torqued/threadlocked properly after my last service.

Just in case no one answers I would have just went to a HD dealer for one - or email them with the pic.
Otherwise can't you pull out another one and measure using a thread gauge from a tap and die set, or take it to Bunnings and use their board gauge (ask someone there) or any shop that sells bolts - engineering supplies etc.

3/8" UNC

 

Found the details I needed:

3/8-16 X 7/8 (Grade 5)
OEM Part# 4266A