J69 - J78 were printed on the flat plate press and J79 - J87 was printed on the rotary press. J69 - J78 are perforated 11x11 and J79 - J87 are 11x10½.
J69 to J78 were issued in late 1930 and early 1931. J69 - J76 were used for less than a year and the replaced with J79 - J86 in 1931 and 1932. J77 was replace with J87 in 1956 and J78 was replaced the J101 in 1959.
J69 - J76 had very low printings. For example, there were 4 plates used to print J75 with only 1000(!!!!!) impressions each. These were 400 subject plates. 4 x 400 x 1000 = 1,600,000 stamps. These were used up very quickly.
I often see eBay sellers and sometimes even professional stamp dealers getting them mixed up. Below are scans of J71 and J81.