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Action is a Steyr 1909 production, 6.5mm London proofs as pre 1925...
...Barrel is roll stamped "Sighted & Shot by W.J. Jeffery & Co. Ltd. 13 King St. St James London SW and serial number".
The 13 King Street address is said to have been closed in 1914...

So.....the date mystery will have to wait for an answer from the business that holds the Jeffery daybooks.

My major discovery is that this 1903 is not a .256 Gibbs Magnum at all......it is a nice unaltered standard 6.5x54MS.

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The 'Steyr 1909' stamp on your left rail indicates a receiver made in 1909 for a Y1903 'Greek Contract' military Mannlicher Schoenauer which would, of course, originally have been 6.5X54 MS.

Other examples:







Commercial MS such as M1900, M1903... had the manufacturer's name in the same location rather than a date.



Remember that for Steyr the Mannlicher Schoenauer was designed to be, first and foremost, a military weapon. The MS sporting rifles and stutzen that we all know and love are descended from a variant of the military type M1900 that were originally shopped to various procurers of ordnance.

For those who'd like a very detailed history of the development of and production changes of the Y1903 and its variants, view this video: https://www.youtube.com/watch?v=KRUxiORNEpE&t=2031s

For the directly relevant bits, the Y1903 is shown at 24:03 and the side rail date explained at 24:42 minutes through the video.

Your 1909 dated receiver fits nicely with a barrel signed by a British gunmaker prior to World War One.

Here's a 'packet loading' Jeffery Mannlicher: