Colts started numbering the slides under the firing pin stop plate with the serial number on the receiver at the very beginning of the Transition series in 1924 but the practice was stopped shortly thereafter. The practice was used with the production of Commercial Government models starting in the C127xxx range after they tried numbering the slides on the center rail on the bottom of the slide. This apparently turned out to be unsatisfactory and they moved the numbering to under the firing pin stop plate shortly thereafter. This numbering practice was carried over onto the first military Transition pistols but was discontinued shortly thereafter. The 1937 contract (starting at sn 710,001) had the slides marked under the FPSP from the beginning. This continued until Colt and the Ordnance Department loosened up the tolerances on the pistols which facilitated discontinuing numbering of the slides since there was no need to keep the slide and receiver matched because the two would completely interchange after the new tolerances were adopted. For quite a while sn 1137399 was the highest serial numbered slide that was known, but fame is usually short lived and someone found a slide in the 11378xx range. I do not know of any documented slides past the 1137900 mark but they could certainly exist and I would welcome pictures thereof. However it is not known if the serial numbers were applied contiguously at the end of the numbering process because the numbers applied under the FPSP were apparently done by hand and not done by an automatic indexing machine as was used for the receiver serial stamping and the last batch of slides that were numbered may have not been completed. Therefore the last observed numbered slide may not be the last contiguously numbered slide, just the last observed numbered slide. I doubt that the numbering goes much beyond the 1138xxx mentioned by DSK because collectors have been looking for the highest new number for a long time, but an example could show up and I would welcome the news and pictures. However anecdotal reports that have no pictures or documentation are really just stories that are subject to poor memories and other distractions.
It's not factory. It looks to me like the slide was drilled and tapped for an adjusting screw for an adjustable sight. A lot of these pistols have been reverted back to OEM appearance after the fact since the values started really climbing.