The company started out as Parker & Ditson in Boston Mass. in 1837 but Oliver Ditson brought out S.H. Parker in 1842 making the name Ditson, then in 1857 with extra directors it became Oliver Ditson & Co. which it remained until it was taken over in 1931. The main business of the company was music publishing and as this was well before any form of music recording music publishers had shops where they sold the sheet music they published and also musical instruments to play the music on. This meant Ditson had a number of shops; certainly one in Boston, one in New York and one in Chicago, (there may have been more), and in these shops along with the sheet music they sold a range of instruments, including Ukuleles, some of which they branded as Ditson. 

Ditson themselves did not make musical instruments, (though as the company that sent George Lyon and Patrick Healy, to Chicago originally and lent them the capital to start their own manufacturing company they may well for a time have been shareholders), they had other companies make them and put Ditson labels on. One of the main suppliers of Ditson branded instruments was Martin, (in fact it was Ditson that was the main reseller for the first Martin Ukuleles of 1906/7 and Diston was one of the two firms that got Martin to start making steel strung Guitars.) Ditson NY got Martin to make a special wide bodied "Dreadnought" Soprano model specially for them however the Boston shop had ordinary shaped Martin Ditsons 

Not all Ditson Ukuleles were made by Martin, they sold quite a few Hawaiian made ones too, some of which were made by Kumalae, (not using one of their standard designs). Obviously Martin wouldn't have made Ditson Banjoleles either; I have seen a Houdlett Nu Art with a "made for Ditson" label on it. I have also read that Victory was a Ditson brand name for Banjoleles and that the actual Banjos were made by Regal though all of the ones I have seen were made by Slingerland.