Gearbox selector to gearlevel bolt?

[tidyclio]

Wondering if anybody knows of the top of there head what sort of bolt this is? Is there a washer? Having dismantled the car months ago and put it all back together recently I think I may have used the wrong bolt. After a few miles of driving lost all gears as the bolt had came off. Put on another bolt and again after about 20 miles today it came off. Have put another one on that isn't the best of fits as it doesn't wind into all the threads though has a built on washer and have put loctite on. Seems to have done the job but would rather have it done right.