Page 289 of Ian Skennerton's "The Lee-Enfield Story"
"British approval was required for such changes and manufacturing concessions to components because a sizeable percentage of the Long branch rifles and virtually all the Stevens-Savage rifles were to fill British orders, and were therefore subject to acceptance by the British Inspectorate. In the initial Savage production, many rifles were rejected by the British inspectors; this was not properly resolved until a Lend-Lease agreement was formulated between the British and U.S. governments.
Under the new agreement the British had less control in the initial inspection and refusal of rifles; conditions for payment were changed also which further reduced some of the controls Britain previously had. The Lend-Lease rifles are marked "U.S. Property" while those produced earlier under the direct purchase system were not so marked as they were for a direct British contract."
As for the rarity of the Mk1s, Savage and Long Branch changed over very early in production to the Mk1*.So a Mk1 from either manufacturer is a neat find.