I first ran into the difference years ago when trying to upgrade the box top for a 1947 K22. Since then, I have been watching and inquiring about every K22 gold box that comes up for sale.
As has been pointed out, it is unlikely that the break was "clean." However, it appears to me that by 1949 production, the early style boxes were gone for sure. I therefore conclude that the early boxes were used up in 1948.
Of course, I am a Colt guy so my information may not be reliable.