I think the "M. Walker" on Ron Cheek's Great Exhibition medal could be Marcus Walker of Boston. He is listed among the names of the attendees in "An Account of the Proceedings at the DinnerGiven by Mr. G. Peabody to the Americans Connected with the Great Exhibition at the London Coffee House Ludgate Hill on the 27th of October 1851" (this publication is available online via GoogleBooks).

The best candidate I have been able to find to be this Marcus Walker would have only been 15 in 1851, however, I do think there is much to support him being the same Marcus Walker as on the "ForServices" medal.

This Marcus Walker was born February 1, 1836 and died September 21, 1887. He is buried in New Jersey. His father, Cornelius Walker, was a graduate of Dartmouth College and was notable inBoston and environs as a grammar school headmaster.

In the 1850 Federal Census Marcus Walker is listed as living with his family in Boston. In the 1855 State Census (age 19) his occupation is "Bookkeeper" and that of his younger brother Marcellus(age 16) is "Clerk". I mention the younger brother's age and occupation as he is about the same age as Marcus Walker would have been at the time of the Great Exhibition.

In 1857 the Boston Evening Transcript newspaper reported that Marcus Walker delivered the closing address at the 8th anniversary meeting of the Franklin Literary Association (the subjectwas "Fame"). The article also noted that he would be "leaving for the west in a few days". In an 1857 Census he appears in Dakota County, Minnesota Territory (occupation "Mill Owner"). Also at thistime it appears that his father Cornelius became involved in mining and was a principal in the "Boston Mining Company".

In 1865 he married Hattie Gladwin of Brooklyn, NY and in the 1870 Census they are living in New York City (Occupation: "Iron Merchant"). Later in the 1870s he was secretary and treasurer of the"Black Hills Placer Mining Company". From 1879-1880 famed writer Ambrose Bierce was retained by the company to oversee the operations.

From Assault on the Deadwood Stage: Road Agents and Shotgun Messengers (Robert K. DeArment 2011), pg. 250:

This company went into foreclosure in 1882 after the superintendent Captain I.M. West "fell in with an unknown beauty and lived with her on a scale of princely magnificence" (The Montana Standard,Feb. 15 1883) spending much of the money that the stockholders had provided for improvements.

The flume works constructed for this mine are presently part of a popular National Recreational Trail in the Black Hills.

To read the "Proceedings at the Dinner" on Google Books, see:
An account of the proceedings at the dinner given by Mr. G. Peabody to the ...

To read the earlier E-Sylum article, see:
QUERY: WHO WAS M WALKER? (http://www.coinbooks.org/v20/esylum_v20n22a15.html)