Actually, it was the MacMillian Baseball Encyclopedia that credited Connor with 136. That was 1980. Prior to that, John Tattersall's Home Run Handbook gave Connor credit for 138 HRs in 1975. Most baseball research databases acknowledge 138 as his career number understanding it may not be 100% accurate. It is also said that when Ruth passed Connor on the all time list, any celebration was technically premature due to Connor being credited with 131 at the time. Based on the time Connor played (from 1880-1897), it is understood that not all the game stats were properly recorded.
The 1890 Players League home run champion (14) ended up having a Hall of Fame career. Connor was inducted into Baseball's Hall of Fame in 1976, well after he should have been. His numbers were worthy of induction, but the time he played is certainly against him. He started his career playing for the Troy Trojans in 1880 and by 1882 had led the National League in triples with 18. He then joined the New York Gothams (who became the Giants in 1885). Through his first 6 National League seasons, Connor had accumulated just 15 homeruns, an average of 2.5 per season. Despite hitting just 1 HR in 1885, it was a breakout season for Connor, who led the NL in batting average (.371), OBP (.435), hits (169) and total bases (225).
He became a "home run hitter" starting in 1886 with the Giants when he finished with 7. From 1887 to 1889, he finished with 17, 14 and 13, respectively. In 1890, he played for the Giants who were in the Players League and led the circuit with his 14 for the season. After playing for the Giants (NL) in 1891, he joined the Philadelphia Phillies in 1892, before rejoining the Giants for 1893 and part of 1894. In what is an unknown transaction, Connor was traded in 1894 to the St Louis Browns, where he finished his career, retiring after the 1897 season.
A .316 hitter for his career, Connor finished with 1620 runs scored and 2467 hits. He had 441 2Bs and 233 3Bs to go with his 138 HRs. He also finished with 1323 RBIs and over 1000 walks to go with his 455 career strikeouts (at the time, the strikeout totals were considered very high). A stolen base was not considered an official statistic (or simply was not counted) before 1886, so Connor probably had much more than his official total of 244 steals. Neither here nor there, Connor also played nearly 200 games at 3B and 2B, interesting considering he was also a left handed thrower.
Though his numbers were not dominant, I find it amazing that MLB did not honor its home run king until 1976, 45 years after his death. The Hall of Fame started its membership in 1936, five years after Roger Connor passed away.