The NVA stamp rang a bell so I searched around a bit. NVA is short for Nationale Volksarmee - the army of the German Democratic Republic (GDR). I came across a German page where they sell the old uniforms, and looking at the collar, the rank sign at the shoulder and the pockets, that looks exactly the uniforms I see there. They also show a photo of the inside of a jacket, also stamped "NVA" and with the size. The other characters to refer to the year and to where it was made. The P1 would according to that mean it was made in the first quarter of 1986. The last code I cannot read - 18?? This would tell you where it was made. The SG48 refers to the size - SG = sehr gross = very tall + size 48 = a tall and slim guy.
It seems though that it has been shortened, and that band at the waist has been added - all the jackets I've seen were longer, like classic uniform jackets, I haven't seen anywhere a picture of a short jacket like this one. What does the construction at the waist look like inside? Any sign that it has been altered?