Falcon - while I have not heard of a 5.56 dummy round from the DDR, it is not impossible by any means. They produced the 5.56 x 45 cartridge in 1988 and 1989, in lacquered steel cases with brass primers, red primer and case mouth seals, and using an SS109 type bullet with lacquered green tip. At first, these were thought to be tracers since the green tip signifies such in most COMBLOC ammunition, and the color was the very pretty, lacquered green of those tracers, not the flat green of most SS109 ammo. Once the rounds became available enough to take one apart, someone discovered they were actually Ball, SS109 type, not tracers. Headstamps are “04 88” and “04 89” respectively.
I also have a rosebud crimped blank dated 88. Aside from lacking a bullet, and therefore also a case mouth seal, the characteristics are the same. The mouth crimp has 8 lobes.
Your dummy has all the look of East German types. The only thing that throws me about it, as to being East German, is that 5.45 x 40 Russian dummies produced in the DDR are not black plastic, but rather from standard live-round materials with veritcal flutes in the case. It seems towards the end they did away with the black-plastic dummy type. this still does not totally rule out the existence of 5.56 black plastic dummies from the DDR.
All that said, I have no real answer for you. The round looks East German, but could be something completely different. Plastic dummies are not unknown from other countries. I am not a .223 collector, although if this turns out to be from the DDR, if you ever find a seond one, would love to trade for it, as i have a fairly large DDR collection of metallic rounds (no shotgun shells - to hard to tell if they are “East German” or “Eastern German” (meaning made before or after the period of the DDR, but by the same factories).
A picture of the head would be helpful. You mention a “snap cap plastic surface.” In the late-type DDR Black Plastic Dummy rounds, this “mock primer” snap-cap surface is an extension of the plastic of the “cartridge” coming down thru a hole in the steel post within the cartridge, to form the snap-cap surface. It would be nice to see what this looks like on your cartridge compared to other known DDR dummies of the same type.