I think that Six-Pointed Star: Its Origin and Usage by O. J. Graham is responsible for the spreading of the 666= Six-Pointed Star. The question is if he had previous sources to lean on. I read many articles that quote him...
There are 1,130,000 results in Google for six pointed star mark of the beast and 33,200 for six pointed star mark of the beast O. J. Graham
Alexander Mann told me that Yan Van Helsing wrote similar things in his second book, but this book was published in the 90th while O. J. Graham published his first edition (out of four) around 1983
Conspiracy of the Six-Pointed Star: Eye-Opening Revelations and Forbidden Knowledge About Israel, the Jews, Zionism, and the Rothschilds by Texe Marrs 432 pages, 2011, ISBN-10: 1930004575
P.S
Lucian Craciun comments
... and here is the proof: old photos of the "Protocols of the Elders of Zion", dating from 1912
to 2005, most of which are asociating the Jewish Hexagram with devil-symbols or death-symbols (the snake, skulls, blood, etc.) and making repeated references to the word "Antichrist". The oldest photo, the one from 1912, has the two 'Seals of Solomon', (i.e., the Pentagram and the Hexagram), and under each one the words: "Pec'at'
Antihrista", meaning "the Seal of Antichrist". (Above the Cross we see the words: "Symb[ol of]
Victory"). The words below I don't know what they mean.