I believe that any Mk I's in arsenal storage between 1939 & 1940 were upgraded to Mk I* specs. The thought behind the initial upgrade was the round could fall off the bolt face prior to chambering. Any and every service rifle available had to be ready for action which is why I believe the upgrade was done. The majority of rifles seen without an upgrade in original Mk I configuration look like they have never been reworked since originally issued. Rifles that most likely were never returned to store. They either remained in private hands or some far off colony post war.
Please see the pictures of my Remington P14 which has under gone a Weedon process.
The mouth of the barrel at the chamber has been relived to accommodate the larger bolt lug. This was the common process to upgrade the rifles with a replacement bolt. Some bolts were new manufactured but most were used and were always matched to the original maker. My bolt in particular was a used Remington and previously numbered. The old serial was ground off the bottom of the handle and forced matching numbers were added to the top. Other than that the rifle was all matching. The tell tale "daisy" indication of a Weedon process refurb is stamped throughout the rifle including the chamber, bolt handle, extractor and stock. - Bill