Hi Tharris,
although i'm very new on this forum, i'm already 25 years in collecting and studiing swords.
The name Sukenaga suggest it is a sword of the Yokoyama school.
The name Toshimitsu was first used by Sukesada V, who was active during 1745 - 1771.
There are also two swordsmiths who are related to the school.
Tos 185, active around 1966, and Tos 187, active around 1954.
So perhaps the reading means made by Toshimitsu in the style of Sukenaga, you should look for resemblence tothe style of the school on your sword.
Tos 185 used itame hada and a o choji hamon.
greetings Yoshii