The M1 was known for "M1 thumb" [1] which typically involved catching your thumb between the bolt and the back of the chamber if you weren't especially cautious. I'm not sure if the author considered this, but it seemed a fitting choice for C. It works and works well, but it can bite you when you least expect it.
Although the source I've linked (below) claims it's impossible to encounter M1-thumb while loading, I have heard from reliable sources who've fired heavily abused Garands in ROTC (and hence saw all manner of abuse--besides WWII) that letting go of the bolt after forcing the clip into the firearm without removing your thumb will correctly strip the first round--and eat your thumb.
Edit: I do have to wonder if the experiences I was related some years ago was from the individual(s) in question loading a single round rather than the full clip.
As a former M1 Garand owner (replaced it with a lighter weapon due to my RSI weakened arms), your link is to my knowledge correct. While I avoided the screw cases where the bolt was not properly locked back, I can attest to other examples, most especially loading a clip finishing with loading the top round. The cartridge is indeed where your thumb otherwise would be, your thumb is essentially pushed out the way, I'm pretty sure it's very hard to get hurt doing this. Or look at it this way, it would have been very sub-par to have a real problem there when soldiers were loading clips while their minds were preoccupied with combat, and back then in the '30s the Army really really cared about the quality of our small arms.
The #1 screw case I knew about pertained to closing the bolt without loading the rifle, there is danger since it requires reaching past the bolt down to the follower that is locking it back. The trick is to hold your right hand in a rigid way so that while your thumb is depressing the follower, the bottom of your hand is holding the operating rod handle, keeping the bolt from flying forward. Depress the follower, then lift your entire hand as a unit up and out. Works like a charm.
Overall I rate the Garand as having better ergonomics than our following two service rifles, the M14 and M16/M4, it's for example very smooth in profile, very little to get caught while you're carrying it (that insight from someone who used it in Korea). If I had one today, and only that rifle, I would not consider myself to be under-equipped.
Hmmm, one other screw case it has in common with all weapons with the common pattern of an operating handle that is linked to the bolt, i.e. reciprocates with it, is that if you have a hang-fire---you press the trigger and the weapon goes click, but unlike the Java example ^_^ there is a round in the chamber---you'd best pull the handle back with your smaller fingers, palm upwards and under the bolt. Because if the round fires while you're doing this, if your palm is reversed and you're using your forefinger and on without taking precautions, the handle will smash into your thumb or the web of you hand between it and your palm. That'll do some serious damage....
C is not quite as good as the Garand is, although I'm not sure what I'd use as an alternative.
Although the source I've linked (below) claims it's impossible to encounter M1-thumb while loading, I have heard from reliable sources who've fired heavily abused Garands in ROTC (and hence saw all manner of abuse--besides WWII) that letting go of the bolt after forcing the clip into the firearm without removing your thumb will correctly strip the first round--and eat your thumb.
Edit: I do have to wonder if the experiences I was related some years ago was from the individual(s) in question loading a single round rather than the full clip.
[1] http://www.garandgear.com/m1-thumb