I was not sure about this one but I did find out some more info from Brophys book, here it is.
I think it may in fact be a real carbine with a replace3ment barrel. The stock is 100% sure a carbine stock, the serial # is in line with Brophy's book info in that here is a section from hs book.

M1898 CARBINE 5000 MANUFACTURED

M1898 CONVERTED CARBINE 3000 MANUFACTURED

M1906//1899 CONVERTED CARBINE, PHILIPPINE CONSTABULARY 5002 MANUFACTURED, used the model 1899 action.

M1899 CARBINE FIRST MODEL, 1896 9000 MANUFACTURED, used the model 1896 action.

M1899 CARBINE SECOND MODEL, 1898 16000 MANUFACTURED, used the model 1898 action-this may be mine here.

M1899 CARBINE THIRD MODEL, 1901 2000 MANUFACTURED, used the model 1899.

This shows me that they used the model 1898 actions all the way up until the end of production.
Attachment 27435