On my SM58 microphone, the collar reads: "Dual Impedance 50 & 150 ohms". It's a tired looking old unit, but a favorite among the singers at our church. It resembles the unit in your spec sheets, but there is no mention of dual impedence. What do I have here?
From 1966 until 1984, the SM58 was dual low impedance: 50 ohms or 150 ohms. 50 ohms is now an obsolete standard, but was used in the 1950's and 1960's for direct connection to some broadcast mixing consoles.
The Shure 1985 catalog lists the SM58 as having only one output impedance: 150 ohms.
For use with modern equipment, your older SM58 should be configured for the 150 ohm setting.