Tabu

It is NOT a makers mark - there was never a brand called Tabu
In the early part of the twentieth century as the Ukulele became more popular there was a general acceptance that Hawaiian make Ukuleles were of better quality than US mainland made ones and as such commanded a higher price. For Hawaiian makers this price was necessary to counter the increased shipping costs of getting them to the US Mainland so anything to undermine this price structure hurt the Hawaiian manufacturers. Of course from the US manufacturers viewpoint protecting Hawaiian Ukulele manufacture was not their concern, so they had no qualms about trying to pass off their products as Hawaiian made, (They often seemed to shoot themselves in the foot here though by misspelling Hawaiian phrases on their labels like Oscar Schmidt with their "Hawaian Ukuleles" and Lyon & Healy with the "Mouna Loa" range).

In order to authenticate Ukuleles actually made in Hawaii from those made on the US Mainland pretending to be from Hawaii, Hawaiian Ukulele manufacturers registered a special trademark and started putting it on their Ukulele's, either on the back of the headstock or inside the sound hole. This occurred from about 1915 to 1925 and is often the only mark found on a old Hawaiian Ukulele. This trademark is called the Tabu Stamp

Not all Hawaiian made Ukuleles of the time carried it, Kumalae Ukuleles for example didn't use it, but any that do carry it were made in Hawaii. The practice of using it also didn't abruptly die out in 1925 so some Ukuleles that carry it may be later than 1925, but by this point it has served its purpose and mainland makers no longer tried to pass their Ukuleles off as Hawaiian made