Dan,
according to my book they both have the same bodies, same lethal radii, same explosive (Composition B). The only difference is in the fuze. The M68 has a M217 fuze, whereby the M67 has a M213 fuze. A M213 fuze is standard on the M68. This fuze has a back up should the electrical Impact fail by not hitting something hard. This back-up is just a normal 4.5 second standard on other grenades. This fuze is very interesting in how it works and has an almost perfect reliabllity score. The fuze is very special on how they could fit such a sytem inside the grenade in order to make it work both ways. NOTE: I do not have a detailed picture to show. Maybe somebody here has one.
Now days everything that is explosive in any way has a safety clip on it.
Also, the M228 fuze is mostly used in the "Practice Grenades" such as the M62 and M69. It is designed to replicate the firing train. This looks to be a standard fuze. The fuze will also have a detonator and a small black powder charge with it. With this in mine it acts exactly like a regular fuze system with a bang. I have seen, however the M69 practice grenade during a training video that they will just use the detonator whereby it makes a loud pop/crack distinct noise.
Also, an example of how a safety clip changes the grenade(s) model is the M26A2. With the clip on it turns it into the M57 with an "Impact" fuze. The safety clips on these grenades M68, M61, M67, are not interchangeable according to the 1975 Jane's book. The 1990-1991 shows the M67 and the M68 using the exact same clip.
Todays hand thrown grenades in the US Army, Navy, and Marines have changed a few times over the last 60 years or so. They are much more reliable and powerful, with an increase in overall lethality than we had in WWII. You could also see why todays grenades are shaped like a baseball. No brainer on this point.
...And like I have stated before, I am no friend to the 40mm being thrown from a M203 system. Rifle grenades are almost always better! Rifle grenades are not affected by weight and the explosive capability and the shape. With "Bullet Traps' now in play a "Mecar smoke and incendiary long range rifle grenade SMK(WP) - RFL-55 BTU M280" series or the Israeli made BT/AP APERS rifle grenades would do nicely for anybody who is trained to use them in the field.
Please excuse my final lines which are just my opinion and nothing else.
I hope this is clear to all of you and please forgive me for any mistakes I may have made. Fuzes can be very complicated at times.
Mark
V40
"Living Historian on MACVSOG"