Wikipedia entry for more).
Burt Alvord (1866-after 1910) was a one-time deputy who turned to armed robbery; Alvord escaped to South America where he was last seen in 1910 (Wikipedia). Bill Bonney was a form of William H. Bonney (1859-1881), aka Billy the Kid (Wikipedia); Wes Harding or Hardin was a form of John Wesley Hardin (1853-1895), outlaw and gunfighter (Wikipedia); Frank McLowery was a variation of Frank McLaury (1848-1881), one of the gunfighters at the OK Corral (Wikipedia); Johnny Ringo (1850-1882) was an outlaw who tangled with Wyatt Earp (Wikipedia).
There were one or two variations: Will Travis was probably William B. Travis (1809-1836), who died at the Alamo (Wikipedia); Virgil Earp (1843-1905) was brother of Wyatt and one of his deputies at the gunfight of the OK Corral (Wikipedia); and Burt Mossman was Burton C. Mossman (1867-1956), a border lawman who captured Augustine Chacon (Wikipedia). The only pen-name he used that isn't related to outlaws or lawman is Mark Reno, although I wouldn't be surprised if this, too, was a variation on a real name. [Update: indeed it is—Marcus Reno (1834-1889), who has a Wikipedia entry here. My thanks to David Ainsworth for pointing it out.]
To confuse matters further, Keevill would occasionally employ one of his pseudonyms as a character in books, thus Clay Allison appeared in a novel by Frank McLowery and McLowery and Virgil Earp appeared in a novel by Will Travis.
Keevill was born in Leicester on 5 October 1914, the son of Walter James Keevill, an engineer, and his wife Beatrice Ellen (nee Sharpe). He attended schools in Aylestone and Leicester before working as a dispatch clerk for the police, a truck driver and electrician in 1931-33. Keevill then joined the Coldstream Guards in 1934, departing in 1937 to work as an accounts clerk with the British Air Ministry.
He rejoined the army in 1939 and became a sergeant, training recruits throughout the Second World War. He was then employed in local government between 1945 and 1960 and the civil service, in London, from 1960. During this time he kept up a steady stream of novels and short stories, writing for the Evening News, Parade, Woman's Story Magazine, Woman's Realm, John O'London's, Weekend, Argosy, The New Strand and various other newspapers and magazines, usually using his Clay Allison pen-name, with the variant Alison Clay used on stories for women.
Keevill was married in 1940 to Gladys May Simpson and had two children, Gillian and Jacqueline. He died in Surrey on 31 January 1978, aged 63.
Novels as Clay Allison
No Rest for Lawmen. London & Melbourne, Ward, Lock & Co., 1955.
Branded. London & Melbourne, Ward, Lock & Co., 1956.
Brand of a Cowboy. London, Instructive Arts (Wagon Wheel W48), 1958?
Gun Gold of the West. London, Brown, Watson (Wagon Wheel W102), Apr 1961.
The Bounty Hunter. London, Brown Watson (Wagon Wheel W110), Jul 1961.
The Drifting Gun. London, Brown Watson (Wagon Wheel W130), Oct 1961.
He Rode Alone. London, Robert Hale (London), 1963.
Outlaw Trail. London, Robert Hale, 1963.
Gunsmoke over Wyoming. London, Robert Hale, 1964.
Guns across the Rio Grande. London, Robert Hale, 1964.
Six Guns for Water. London, Robert Hale, 1964.
Six Guns in Sundance. London, Robert Hale, 1966.
The Wandering Gun. London, Robert Hale, 1966.
North from Texas. London, Robert Hale, 1969.
Trail of the Iron Horse. London, Robert Hale, 1971.
Novels as Burt Alvord
Gunfighter Breed. London, Robert Hale, 1965.
Novels as Bill Bonney
Colorado Gunsmoke. London, Robert Hale, 1964.
Novels as Virgil Earp
Hatchet Rides High. London, Robert Hale, 1964; New York, Arcadia, 1969.
Novels as Wes Hardin
Trail from Yuma. London, Arthur Barker, 1956.
Novels as Wes Harding
Trouble at Gunsight Pass. London, Robert Hale, 1963.
Gun Law in Toledo. London, Robert Hale, 1964.
Cattle Country. London, Robert Hale, 1965.
Novels as Frank McLowery
Missouri Man. London, Macdonald & Co., 1955.
Herds North. London, Macdonald & Co., 1956.
Guns for the Sioux. London, Brown Watson (Wagon Wheel W91), Jan 1961.
Novels as Burt Mossman
Guns on Eagle Creek. London, Robert Hale, 1964.
Novels as Mark Reno
Stagecoach to Fremont. London, Gresham, 1965.
Novels as Johnny Ringo
Lonely Gun. London, Mills & Boon, 1954.
Action in Abilene. London, Mills & Boon, 1966.
A Gun for Vengeance. London, Mills & Boon, 1966.
Novels as Will Travis
Lawman without a Badge. London, Wright & Brown, 1968.
The Hangrope Trail. London, Wright & Brown, 1969.