1869 Bayonet: https://www.rockislandauction.com/detail/72/134/scarce-u-s-model-1869-trowel-bayonet 1873 Bayonet and Rifle: https://www.rockislandauction.com/detail/72/151/u-s-springfield-model-1884-trapdoor-rifle-with-trowel-bayonet
The United States first experimented with a combination trowel and bayonet in 1868, producing 200 experimental examples made from standard socket bayonets. This was immediately followed by an additional 500 Model 1869 trowel bayonets made new. These were distributed to a few companies of the infantry to test in the field. Remarkably, the trials reports were overwhelmingly positive.
The US infantryman at that time did not carry any sort of entrenching tool, and so even an awkward combination tool was an improvement over a canteen cup or other ad hoc tool for digging. The bayonet was seen by some officers as becoming obsolete with the introduction of breechloading rifles, so the reduced effectiveness of the new item as a bayonet was not a substantial concern. The intended use of these tools was not to dig elaborate trenches, but rather to hastily construct a shallow ditch and embankment which would provide just enough cover to shelter a prone soldier.
With the trials reports in, the government purchased 10,000 of the improved 1873 pattern trowel bayonet, which featured a stronger blade and a much more comfortable handle for digging. These were issued and used in the field (and in several combat engagements), but the developmental direction turned towards combination knife trowels instead of bayonets, and there would be no further development or issue of these tools after the 1870s.
See the full trials report here: https://books.google.com/books?id=qUEqAAAAYAAJ&printsec=frontcover#v=onepage&q&f=false
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