The M-40 has five buttons, as does the M-36... (I think it's just a wierd wrinkle in the fabric that's got you confused.. ) The only external difference between the two was the removal of the Bottle Green collar and Shoulder Marks from the M-36... However, the Bottle Green collars and shoulder marks were popular with the troops, and it was authorized as an addition, so many M40-issued Soldiers had them added at tailor-shops... Without seeing the lining, it's hard to tell the difference.. The next new tunic was the M43, which did away with one button, the pleated pockets, and the scalloped pocket-flaps, externally, and had changes to the sewn-in suspension system...
The "Marching Boots" (Jackboots) stayed in service until the end of the war, but the low-cut, gaitered "retreat boots" were issued to new troops, and to veteran troops as their jackboots wore out, starting in 1943... However, lotta "old guys" had more than one pair of jackboots, and you still saw them at war's end...
The two Unteroffiziers (Corporals) are easily ID'd as NCOs due to the silver Tresse on the collars and shoulder marks... I'd guess that that photo was taken in the late Summer of 1940, that's a SWAG based on the uniforms' Lichtgrau collar Litzen in combination with the trees....