I'm sorry to say that it is a counterfeit. It's got the wrong sleeve stripes for a 1975 Bears jersey. George Halas was alive at this point (he passed on Dec 16 1979), the GSH wasn't added to the sleeves until after his death. The neck is wrong, in 75 it was a rounded neck, not V necked.
Here's the link to the M&N website:
http://www.mitchellandness.com/Products/Chicago-Bears-1975-Dark-Jersey---Walter-Payton__7220-200-75WPAYT.aspx