Some doubt to this is given due to the knowledge that my grandfather was not part of the 'Anders Army' imprisoned in the USSR, but joined the fighting on the side of the Allies after deserting from the Wermacht.
Your doubt is unfounded, even though your grandfather was not a part of original “Anders Army” it is precisely this army that grew in numbers due to the large influx of Poles serving in Wermacht that either deserted and switched sides or were captured and switched sides. These were the Poles mainly from Silesia, Greater Poland and Pomerania that were forced to serve in the German Army as conscripts. These were the parts of Poland that were directly incorporated into Germany.
" 3 divi Kotpock Sivis "
I can’t help you solving this mystery; I simply have no clue as to what it might be. Perhaps if you made a picture of it and posted a link to it here someone might be willing to translate the hand writing.
This might be of some use in your research. Polish II Corps under General Anders was part of British 8th army. When it was formed II Corps consisted of two Infantry divisions, one panzer brigade, one independent altirely unit and couple more smaller units. Since it was growing in numbers throughout the campaign, mainly because the former Wermancht soldiers were joining the ranks, new units were established as part of II Corps. These were;
3rd brigades in both of the original infantry divisions (this most likely is the place where your grandfather served, so search for either one of them), created few more units of altirely, 2nd Panzer division became much larger and renamed as 2nd Warsaw Panzer division due to the new recruits and creation of a new 14th Greater Poland Panzer brigade