I would like to find an authentic, commercial Mackinac island fudge
recipe just as produced in the fudge shops on the marble slabs.  I'm
interested in the actual recipe used to make the commercial sized
batches of fudge.

I would recommend you check this website
http:\\grocerybook.vandvinc.com
they are give away a recipe software with access to 30,000 recipes.
You are bound to find one in there.

The following site also has a recepe for the fudge. but it is not for
comercial size but I guess you can multiply it to meet your needs.

Sorry I forgoyt to give you the second site here it is
http://gamesbygrube.freeyellow.com/mift.html

Hi there! I would recommend getting your hands on the book titled:
Oh Fudge!: A Celebration of America's Favorite Candy as it has the
Mackinac Fudge recipe in it. You can see more about this book at
Amazon.com:
http://www.amazon.com/gp/product/0805025464/104-9642841-1280753?v=glance&n=283155

MACKINAC ISLAND FUDGE 
1/2 C. milk 
1/2 C. butter 
1/2 C. brown sugar 
1/2 C. granulated sugar 
1/8 tsp. salt 
1 tsp. vanilla extract 
2 C. confectioners? sugar 
1/2 C. nuts (optional) 

Mix milk, butter, brown sugar, granulated sugar and salt in heavy pan.
Cook at medium heat until boiling. Boil exactly 6 minutes, stirring
constantly. Remove from heat and add vanilla extract and
confectioners? sugar. Beat with mixer until smooth and thick. Add
nuts, if desired. Pour into a buttered pan and freeze 20 minutes. Cut
into pieces. Makes approximately 1 pound of fudge.

Peanut Butter Mackinac Island Fudge Reduce butter to 1/4 cup and add
1/2 cup peanut butter.

Chocolate Mackinac Island Fudge Use basic recipe, adding 1/2 cup cocoa
with the confectioners? sugar. .

Reference: Lee Benning's Oh Fudge: A Celebration of America's Favorite Candy