K, had a look.
1899A Short Rifle in 38-55, ser 81### has the dot, 1899C in 38-55 ser 91### has no dot.
I'm thinking that when they made the change at 90,000 to the shorter upper tang that they must have done something with the serial numbering machine and took the dot out at the same time. I know some say that the heat treated receivers started at 90,000 as well but it's been proven that this started at the 120,000 range. Either way a lot of differences started around 90,000 so I guess this was one of them.
If anyone has an 1899 between the two serial numbers I gave feel free to share.