The six digit serial number is stamped on the back of the headstock. Note there is no 'MADE IN USA' stamp beneath the serial number, despite this bass being shipped in 1970. This is usually cited as proof of a pre-1970 bass; this is one of the last instruments made by Gibson without this. See a slightly later 1972 EBO with the 'Made in USA' stamp here.
The posts of the Schaller M4C clearly show how the strings pass through the post holes rather than into the top as with the Schaller M4.