I also received this excellent letter:

Subj:  M1866/75 Argentine Remington
Date: 01-03-22 14:32:44 EST
From: eahull@earthlink.net (Edward Hull)
Reply-to: eahull@earthlink.net
To: KeithDoyon@MilitaryRifles(.)com

Hi Keith-

    Saw your update on the "M1866/75 Argentine Remington" rifle.  The excellent photo of the breech/frame, sans forestock, clearly shows that the frame is/was of the type used on the Rem. .58 cal. "transformation" or breechloader conversion muskets. If the breech block is concave (to fit under the barrel) then it is a pre-1871 made frame.  If the breech block is flat and the barrel bottom is cut to accept the block, the it is post-1871 type.  Note that there was a hole at the front of the trigger guard where the original sling swivel was attached--in the later alteration it was plugged and the swivel added to the buttstock. The fact that it has the long trigger guard tang with the screw is further indication that it is the conversion frame, as is the style of breech block.

    Given the rebarrel in Belgium, I wonder if this isn't a surplus French (Franco-Prussian War) frame? It could certainly be of the mid 1870 time period--or much later. Since the Argentines imported all kinds of "surplus" (US CW as well as from other sources) it isn't unusual that such a rifle would end up there. But proving that it is an "official model" or issue arm might be a bit more difficult.
    Keep up the good work.

Ed Hull