Correct. A 6 mm thread.
The original nuts are a little unusual as they have an 8 mm spanner size (the 5/16" mentioned above is very close to 8 mm) where most 6 mm nuts have a 10 mm spanner size.
So to match the other nut you need a 6 mm flanged locknut with an 8 mm spanner size. If you want it to match you might have to buy an original..