The flat blocks work on single action revolvers with stout frames. Given the modest torque on J frames, they can also work pretty well for them. Note: leave the yoke in place while doing J frame with that type of wrench. Use lead sheet pads.

The K, L & N frames need the blocks contoured to the frame. Look closely at the fit of the block to the threaded frame boss. You want an angled flat that engages the flat where the top of the yoke goes so that the torque is on the threaded boss. NOT on the frame on the yoke cut out.

I've got a set of the original MGW blocks for the N frame* and they used a tang that contacted the frame in the cut out on the flat for the yoke and used that as a lever to resist the torque. I've never used them. Anyone who wants them can have them cheap. Now, it's OK for the blocks to contact that flat, but it shouldn't be the primary means of resisting torque on the frame. If that top picture is a current set of MGW blocks, they might have changed the design. Check the fit.

I may be a bit opinionated, but back in the day I did a lot of N frames and some of them apparently used a lot more torque on the barrel than really necessary. [Not lined up, a couple more grunts on the wrench and it is!] I wasn't about to be buying anyone new frames, so I made my own N frame aluminum blocks. It was a nightmare, but I got what I wanted. Armorer951 is right, if you can find correct blocks buy them.

I suppose you could cast a filler out of epoxy to properly fill in poorly machined blocks, didn't think of that back when.

* This was before they added the bar at the bottom of the wrench to prevent spreading. I had to cut the blocks down after they came out with the bar.