Your Spencer carbine serial #55318 is one of those serial #s that really doesn't fall really close to serial #s already listed in my SRS books. So read my disclaimer and then I'll get down to business.
First of all, you really have to read and understand my standard disclaimer that I print out for everyone, who requests Spencer serial # information. So, here it is:
Some time ago I acquired the four volume set of the Springfield Research Service (SRS) Serial Numbers of U.S. Martial Arms. I purchased the set so I could maybe help SSS members identify which unit their original Spencer Carbine or rifle was issued or assigned to during, and perhaps, after the Civil War.
Unfortunately, not all the Spencer serial numbers are listed in the four volume set I own. And, to make identification even more difficult, there are huge and multiple gaps between indicated serial numbers. This paragraph is a DISCLAIMER of sorts. As a courtesy to SSS members, I am happy to reveal to you the information I have regarding the serial number of your Spencer carbine or rifle. But, please remember, I am just regurgitating what I read in my SRS volumes with some "unscientific conjecture" on my part. What I do is the following: I take your Spencer serial number and try to place it in between the two closest serial numbers I find listed in my SRS volume(s). In most cases, if the serial numbers for the Spencer before and after your own serial number were issued to the same unit, I assume your Spencer MAY have been assigned to the same unit. Or, if your Spencer serial number is very, very close to another Spencer serial number in my SRS volume(s), I assume your Spencer MAY have been issued to the same unit.
I call this “unscientific conjecture” the “Two Flints guesstimation”. I’m guessing and estimating at the same time on what I believe to be correct information based upon the information I find in my four volume set of SRS. I certainly and absolutely make no claim to the accuracy of my "unscientific conjecture" and you should use the information I offer only if you want to, and not to accept it as “Gospel”.
Other SSS members and even guests of SSS may post a comment suggesting that my serial # information is completely inaccurate, but haven’t I already done that with my disclaimer?
In any case, the serial # information I provide is only a starting point, at best, and you should perhaps research the history of your Spencer Carbine with whatever resources you can find on the Internet or else where.
Now, regarding your Spencer Carbine serial #55318, according to my SRS Volume, I find that your Spencer may have been issued to either a member of Company E, 6th Indiana Volunteer Cavalry in July of 1865, or to a member of Company D, 6th Illinois Volunteer Cavalry sometime in 1865. I like the second option best!
Hope this helps.