Breech block - need help identifying

[smittyc7]

Can anyone identify this Spencer upper breech block?  I am assuming that there was a spring of some sort on the firing pin.  There is a slot cut into the block that extends all the way through to the other side.  I have no clue how old or modern it is.  It was in my original when I bought it, but as you can see, it is a centerfire.  The more I learn about Spencers, the less I seem to know...

[Herbert]

Looks to be a home made go at making a center-fire block,I would not use it,no support of the base of the cartridge over primer

[Blair]

smittyc7,

Are there any other marking any where on the firearm?
Is it a Rifle or Carbine?

FYI, it is not a variation I have encountered. But then, I too am still learning as well.
More info from you is needed, and even with that it may still end up being a Mystery.
My best,
Blair

[smittyc7]

It was in my original carbine, traced by Two Flints:  An exact match for your Spencer Carbine, according to my SRS BOOK.  Your Spencer Carbine serial # 30337 was issued to a member of Company G, 6th Indiana Volunteer Cavalry on July 13, 1864. 

But since it is a centerfire upper block, it came later.  I was wondering if anyone recognized the make and if it is suppose to have a spring mechanism of some sort to return the firing pin.  It is just loose now, and slides back and forth freely.