Good luck!
I searched and searched for that original part and no way. Asked to the seller for that pin, (who lives in Birmingham Englandicon), and sent to him a picture to identify it (maybe would have one forgotten in his stuff and classified as unknow part)) and no success, so I made a "repro".
This is a pic of the sight attached to my M1917 Winchester. Please note that the T-Pin is hidden under the bolt stop spring..


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Regards
J.