November is done, All Y'all go here, Thankee Kindly

https://www.cascity.com/forumhall/index.php?topic=68421.new#new

The picture you show is a rimfire breechblock--the firing pin is that flat piece that slides along the right side of the block.  On your photo that is the rounded piece peeking out from underneath the block.

A centerfire breechblock will have a hole in the center of the breechblock.  Those can sometimes be tough to see without disassembly, because the cartridge guide (or whatever it's called) is in the way and the block drops down where it's hard to see.  It can also depend on how the conversion was done.  I have attached a crude photo showing the two firing pin locations.  The dot is approximately where a centerfire firing pin would poke out.

The Stabler cutoff is a different animal.  It served the same function as the Spencer cutoff, but is mounted on the bottom of the receiver.  It vaguely resembles a wing nut.

I have not heard of many Spencers being converted to centerfire by the government in 1871 or any other time.  There may have been a few for experiments, but I don't think a lot of them.  The Belgians made some centerfire copies in the 1870s.  Or you may be thinking of the 1871 conversions of Burnside-manufactured 1865 Spencer carbines that were converted into rifles, but those remained in rimfire.  Your best bet is probably a Dixie block.

It looks like a decent gun.  Post more pictures when you get it.

That breechblock has a Lane extractor--in my experience it worked a little better with Starline cases than the Spencer blade extractor.

You should also note that the rifle you are showing has the Spencer cutoff--it's that wide forked cartridge guide on the top of the breechblock.  The top part of the "fork" pivots to fill the gap in the fork and that keeps the breechblock from opening all the way, so cartridges won't feed from the magazine.  The reproduction breechblocks I have seen are not made to fit a Spencer cutoff; I did manage to modify one to work, but it involved a bit of trial-and-error labor to widen the groove on the top of the block.

(Edited to specify improved performance was with Starline cases and again to fix silly typo)

Apologies for the late reply. Thank you for this confirmation and the new bit about the Spencer Cutoff. I looked the cutoff up on the internet and found something called a "Stabler Cutoff". Is this similar ? Also how do I tell if this is a rimfire or centerfire breech block. I read that a lot of these rifles were converted to centerfire in 1871 by the U.S. Government. If this rifle isnt centerfire I was going to buy a breechblock from Dixie Gun Works.

Heavy rain all day here, never got out of the 40s, so an unpleasant day. LadyC and I took it easy and stayed indoors as much as possible. Overall, a good day for hot coffee and a good book.

Based on the serial number and cavalry markings, I can provide some historical details on your 1865 Spencer carbine:  _{^{geometry dash}}
- The serial number of 3894 falls squarely into Spencer production in 1865, the final year of the Civil War. Approximately 13,000 carbines were made that year.
- The "Co K 9 CAV" stamping indicates this carbine was issued to Company K of the 9th Regiment California Volunteer Cavalry.
- The 9th CAV served in the Arizona and New Mexico territories late in the war patrolling for Confederate sympathizers and outlaws.
- So your Spencer likely saw action on frontier cavalry missions under Lieutenant Colonel William McCleave and Major Emil Fritz.
- The "Stabler cut-off" is a unique modification specific to the 9th CAV's Spencer carbines only, ordered by Colonel McCleave. It disabled the hand-cocking lever for safer handling by cavalry troopers.

With all due respect, a correction about the Stabler cutoff: There was no "hand-cocking lever". The idea of the Stabler cutoff was to prevent the action from opening far enough to feed a round from the magazine, so the arm could be used as a single-loader, supposedly to conserve ammo, until rapid fire was necessary. As on all Spencers, the hammer had to be manually cocked after the round was chambered.  In fact, the safe procedure for firing a Spencer is to half-cock the hammer, work the lever to chamber a round from the magazine (or after single-loading a round), pull the hammer to full cock and fire. But, in a firefight situation, the hammer could be full-cocked before working the lever, and the trigger squeezed to fire. The reason for half or full-cocking the hammer, is to prevent slam-firing, as the rimfire fireing pin is longer than the breechblock. With the centerfire conversion, I set the firing pin up so it was an inertia pin, like a M1911A1. Just have to be careful not to shorten the pin too much!
BTW, the 9th Cav and the 10th were the Buffalo Soldiers!