The Tektronix Type J is a 35 MHz dual-trace plug-in for 500-series scopes which never went into production.
The Type J's tentative announcement was in 1963. It appears to be similar to the 1A1, which was introduced in 1964. The 547 training guide, dated December 30th, 1963 mentions "a new dual trace plug-in, the 1A1 (also known as the J)." Note: The Type J does not have a spoiler switch hole in the rear panel, while the 1A1 does have such a hole.