AC Milan Kappa Shirts
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Mostly, there are a few differences between
player issued and replica/fan shirts. One of the
difference could be that long sleeved shirts are
not sold commercially while short sleeved
shirts are.
I often read this argument when old 1988/89 or
1989/90 AC Milan shirts (made by Kappa) are
sold on eBay or other platforms.
But fact is that - as far as I know and remember
- at the time in fan shops there were sold
long sleeved
shirts and no short sleeved shirts.
Therefore, if you buy a short sleeved of those
Kappa shirts, the chance  that it is a player
issued shirt is already high.
Another detail to watch at seriously is the
shape and the exact measure of the
back number, which wasn't sold or embossed
in shops, but only at the kit stock of AC Milan.
There were and always are many fakes sold
around in the internet, often offered as "match
worn" etc., which they simply are not. Once you
get aware of the specifications of the real back
numbers, you recognize them at once.
In 1989/90 there were a few more differences
between player issued and shop replica shirts:
One of them  is the
bigger European Cup logo
on the player shirts, which was embroidered
below the yellow star.
Further, some long sleeves shirts were
produced of a
heavier cloth material to meet
the cold temperatures in winter.
Last, but not least, some shirts had a different
colouring of the collar, which was also never
produced for commercial sales.