'Local Boy' - Great job of spotting that picture of an identical receiver in Mallory's "KRS".

Frank Mallory may have been correct about this 'non-serial numbered' receiver being a component of a 'Sub-caliber Attachment' for Artillery practice. However, I think there are some inconsistencies and errors (which I will explain). 

Both the Army and Navy had sub-caliber devices. The Navy had a device that could be externally attached to deck guns. The Army device was inserted into the bore of artillery pieces. Both systems allowed practice by firing specially prepared .30 caliber cartridges.

Mallory reports Subcaliber Attachments were made at Watervliet Arsenal, using some rifle components supplied by Springfield Armory. Apparently 200 devices were prepared, between 1901 to 1909, for artillery ranging from 3.2" Field Gun to 7" Siege Howitzer.

William Brophy, in the "The Krag Rifle", (pages 206-207), shows detailed pictures of the 'Navy Sub-Caliber Rifle' ("Mark I Mod. 1 Sub-Caliber Rifle"). The device could be solenoid or manually fired, and was attached externally to Deck Guns. The pictures show a regularly machined Krag receiver, with serial number, and a regular trigger which the solenoid plunger strikes. Also, three holes are described as being drilled into the receiver ring to allow the action to be held in the device by lock-screws. The pictured device was made in 1919 by the Boston Scale and Machine Company

Brophy also shows the 'Army Sub-Caliber Rifle', which is clamped into a large bronze collar with interrupted threads, that allow it to be 'screwed' into an open cannon breech. The Krag action is fired with its conventional trigger. The left-side of the Krag action is not shown so I cannot tell if there is a serial number or complete machining.

The picture posted by 'Local Boy' shows a Linkage that I believe to be totally unrelated to the Artillery Sub-Caliber Devices. The Linkage is for a "Cummings Sub-Target Gun Machine". But that's another Story

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