No, the front gate is not the only difference (I have a M104 and a M105 in the yard right now...)
The M104 has a front gate, and required a different front landing leg than the M105 in order for the front gate to sit flat when open. The front landing leg on the M104 does not come above the frame whereas the M105 version does. (anyone have a spare M104 landing leg they'd like to part with??).
The M104 used double ended hoses and electrical line to connect to the tow vehicle. The M104 has a pair of glad hands and a trailer electrical socket on it that is identicle to the glad hands and trailer socket that is on the back of a M35... Loose enough hoses and intervehicular cables, and you switch to the style that is on the M105 where they are affixed to the trailer...
M104 also ran on 1100x20 tires in order to be utilized behind the GMC and REO designed dueces with the single tires on the rear axles instead of the trucks with the 900x20 duels such as the M35...
The hub assembly is also a tad different on the M104 and the center of the hub sticks out father through the wheel than on a M105 so changing tires on the M104 is a little easier...
Oh yeah, of course the wood racks on the M104 are slightly different since it has a front gate... but the sides and rear are the same...
Thanks for the links...