You have the advantage here since you have the barrels in hand. The second letter looks to be "L." What is the first a "J." or an "O." perhaps? How low a serial number is this gun and how is the top rib marked? Are there other stampings forward on the barrel? Are there any British proofs for example?
The initials could be the original barrel makers mark or the barrel strikers at Parker? Is the top rib marked "Laminate"? If so could the initials be for laminate barrels since Parker used various steels and would need to have a way to identify them before blacking? Originally barrel steel was sourced from England I believe except for plain steel and decarbonized steel.
Others could offer more help if we could see the elephant and not just its trunk peaking out from under the tent.
Erick

Hi Erick,
I will post photos of the entire gun tonight. The serial number is 5670 and the top rib is marked "plain steel". There are other markings; these were the most difficult for me to identify, which is why I have the photos on my phone with me all the time. When I return home, I will take pictures of the gun in its entirety. Thank you for the info.
Kerilynn

Is this gun marked parker bros or parker?

It says "Parker Bros. Makers, Meriden, Conn, Plain Steel"

i have such a gun marked as yours about the same serial number also made in 1875....charlie

Hi Charlie,
That's great! Do you know what these markings are, or does you gun have these markings as well? I understand that there weren't many with plain steel on the rib, which has made it more difficult to identify these codes. This is the first Parker I have owned, so I'm learning quite a bit. I tend to collect military and wartime pieces mostly.

i do not know what these markings are..but mine is marked like yours...the plain steel barrels are pretty rare not to many made with plain steel wrote on them..i have 2 such marked guns...they shoot ok with the plain steel barrel with light field loads...charlie

Erick, here is the rest of the elephant from under the tent.;)
Seems I can only upload one pic at a time. I'll post another in a different reply.

Another photo. A bit dark, but reads Plain Steel.
Kerilynn

Kerilynn, Your 2 most recent photos show a decent 0 grade early Parker minus a hammer and hammer screw. Being a plain steel, gun it is my understanding it wouldn't have foreign sourced barrels . Hopefully others with "Plain Steel" barrels or other barrels of the same period can comment on the initials partially covered by the bottom rib. I don't remember my 2 somewhat earlier lifters having any similar initials but I don't have them handy at the moment. The Parker Story lists some early Parker employees and their job functions. If the initials can be identified, someone with the book handy might be able to match them to a barrel man at Parker B. or not.
You can check to see if a research letter is available for your gun by visiting the home page. One is probably available since the stock book for this serial range is available. Don't know how extensive the info in a letter might be until one is ordered.
Erick