I personally think that is pure mythology. My reasoning comes from the fact that all commanders from 1949 through at least 1972 and possibly beyond have those lightning cuts. All National Match pistols have them. The Gold cups from 1970 on past 1972 also have them. All conversion kits and SM Ace pistols have them. I've carried a 1952 commander since around 1973 as a Leo back up and have shot somewhere around 30,000 full power rounds through it and it isn't cracked. A very close friend of mine bought his new in 1954 and has carried it ever since. He and I have shot at least 50,000 through his and it isn't cracked either. As a matter of fact, he loaned it to me for a backup when I started in 1972 and I used it for that purpose and an off duty carry until I obtained my own. Some have cracked and I have seen a few in the years when I was building IPSC guns and carry guns from around 1978 up through 2002. But the ones I have seen have all been commanders and the frames were also cracked from the slide stop hole forward at an angle up toward the front of the slide rails. I have seen a couple of NM slides that had a small crack under the extractor slot. But those were as slides for sale at gun shows, stripped. All total I have actually seen 2 slides and 2 commander frames cracked. Of interest is that both of those had shok buffs in them to which I attribute the frame cracks. Considering that I have worked on and built well over 1000 guns personally and own a considerable number in my collection including 9 guns with light slides, I have to believe that while it does happen once in a while, I don't think it is any more than rare. Also, my personal thoughts are that the use of shok buffs by people who want to shoot high powered ammo for defense has been responsible for a lot of the battering that I have seen in Colts. The shok buff shortens the cycle by allowing the slide to contact it prior to the end of the cycle. This increases the duration of the impact on the frame. And while it makes the total impact softer, it does make it longer. Like an earthquake, it is not the level of the quake that does the damage but the duration of the quake which causes things to crumble. From the standpoint of the potential for catastrophic damage to a pistol, I personally avoid shok buffs and I would not knowingly purchase a pistol that had had one in it. The light slides by the way are not only lighter where the circular mill cuts are made on each side of the disconnector boss but are also cut out and thinner from the area behind the barrel bushing to the front of the locking lugs. If full power loads were the cause, the slides should be cracking forward of the locking lugs through the body. But they aren't. The cracks that are found are immediately below the extractor where the slide is very thin and it is the weakest point. The ejector side is also cut out but the slide never cracks there. Keep in mind that they are heat treated after they are machined. I know there are lots of shok buff equipped guns out there that have not cracked. That's great and I'm happy for the owners. But there are some owners who are not happy. The frame was never intended to take the pressure of the slide recoiling. The end of the spring guide shouldn't take it either. The total of the coiled springs pushed to the rear should prevent contact. But installing shok buffs then alows that impact to be transferred to the frame. A shorter or weak spring would also allow the same thing to happen. Keep in mind in National Match pistols and Gold Cups post 1970, they have a slightly shorter and lighter recoil spring and a shorter and lighter mainspring. If one shoots full power ammo without changing out at least the recoil spring then one was at the time asking for a problem. The closest thing to a shok buff Colt ever used were the leather washers that were installed on the spring guide of original Ace pistols. The washers were nearly an inch long in total. These guns also had a shorter recoil spring. I haven't owned one or ever examined one to see how well that system worked.Before you shoot it also field strip it and check the bottom of the slide. The slide looks like it came off 1966-1970 Gold Cup National Match so it should have cut outs at the rear. You can see the extractor for example during a field strip. I would not shoot hot 45ACP loads in it if that is the case and if a previous owner did then there may be a crack in this area.