I've seen a lot of threads in various forums over the years about the origin of a '1900' box. I found this recently:
"Many people believe a 1900 box got itís name from the fact that it originally was 19 cubic inches. In trying to track that back, I couldnít find any proof of that but the first reference I saw to 1900 was in the 1917 Central Electric Supply Catalog where they referred to these boxes as 1900 Combination Gas and Electric Boxes. The name seems to have come from the part number given by the Bossert Company. One means of early wiring was to use abandoned gas pipe as the wireway and the manufacturers offered products to accommodate this need. Since this was so common early on, they got the name Combination Gas and Electric Boxes. The gas connection was accomplished by using the 1/2" knock-out and closing the cap with a 3/8" flange on the box cover. The image above shows a section from the 1917 catalog describing these boxes. It shows part numbers for the generic LIST#, Sprauge Manufacturing, The Bossert Co and Chicago Fuse respectively." Author unknown.