50ESL74M Tachometer

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I just bought a boat with a 1974 Johnson 50 on there and it is running great. The motor is not the original motor from the boat and they never installed a tach on there. I have read a little about 6 pulse, 5 pulse, etc but am confused as to where I would find out what my motor would support. Does anyone know what type a 1974 Johnson 50 (50ESL74M) is so I can buy the correct tach?

Thanks in advance!

-Steve