According to "American Military Equipage" by Frederick Todd, some "Colt rifles" appear to have been delivered to the Baton Rouge Arsenal in Louisiana prior to the war. See below:
"The seizure of Baton Rouge Arsenal greatly improved the small arms position of the state (of Louisiana) since it secured thereby, among other supplies:
1,099 M1855 rifle muskets
972 Rifled muskets, caliber .69
29,222 Percussion muskets, new and altered
8,283 Flintlock muskets
2,158 Harper's Ferry rifes
73 Colt rifles
2,287 Hall rifles
735 Carbines, various patterns
2,075 Percussion pistols
468 Colt revolvers
The Military Board at once began to issue these arms to the Volunteer Militia …"
So in answer to your question, it seems probable that some or all of these 73 Colt revolving rifles were indeed issued to an early Louisiana militia unit or units .
While the itemized "Colt rifles" are not described precisely as Colt revolving rifles, Colt revolving rifles did in fact take a sword bayonet. So the information in your letter seems to dovetail with the extant evidence, making it possible (if not probable) that some Louisiana militiamen were armed with this weapon.
P.S. To my knowledge, Colt revolving rifles were never referred to as M1855 rifles, as you call them in your query ... even though arm was first produced in 1855, and first issued in 1857.