My 68 DT1 rear brake backing plate has a flat place about inch long. It has a threaded hole with a round head machine screw in one hole, a plastic plug in the other. Anyone know what this was fore/

Could you post a photo or two? "A picture paints a thousand words!"

It's been suggested before that the plastic plug , when removed , allows you to inspect
wear on the rear brake shoes .
I've never actually checked this out on my own bike , so can't vouch for whether this actually works .

Here's one, & No, i don't know what its for, I've been looking at one since 1968 & never wondered. But now i have to know haha.Here's one, & No, i don't know what its for, I've been looking at one since 1968 & never wondered. But now i have to know haha.

Interestingly--or not-- the first model didn't have the bonded rubber steel bush in the anchor arm part of the brake plate--which i knew but had long forgotten. We we were out trail riding in "69 & my bosses December "68 lost the anchor bolt [luckily i'd not had his wheel out lol] & the brake arm from the cam got a perfect horseshoe bend in it as the lot wrapped around the axle. Anyway--not helping am i.

Interesting.  Sure someone knows. 

P.S.  The "shoe wear check" plug didn't show up for a few years...  it became a US requirement around the mid-70's for street legal models.

Here’s a pic of my ‘68 backing plate just after tear down with the screw and plug in;

 

Heres a few more prior to reassembly;

 

 

 I know, I know this still doesn’t answer the original question…

Part number details of the plug on CMSNL suggests that it was first used on the 1965 Japanese Domestic Market (JDM) U5.

Maybe, therefore, it relates to something that was a requirement on JDM bikes only (like the speed warning light was) and was carried over on to the US bikes.

My understanding is, at that time, most new Yamaha models were sold on the JDM before anywhere else.

Sorry, still doesn't answer the actual question, though! 

might be to screw a thingy onto that sits in the spokes & makes a noise like i had on my pushbike as a kid--Ok not that.

My guess from the location is it was there for a cable actuated brake mount.  Not needed for the rod actuated brake used.