Standard AP round. Remember most of the specialized stuff was rare and issued in very small quantities. The standard 17lb'er AP projectile penetrated 151mm at 1000 yards and 129mm at 2000 yards. It is the best AP gun in AH.
Well, statistics can be deceiving, as the state for which they apply is often not specified. However, from the book "Sherman Firefly", by Hayward it specifies penetration of homogeneous armor at 30 degrees as follows ("...based on primary data from PRO documents listed in the bibliography...:
17pdr APC 111mm at 1000m and 87mm at 2000m
17pdr APCBC 124mm at 1000m and 108mm at 2000m
17pdr APDS 172mm at 1000m and 153mm at 2000m
Panther APCBC 100mm at 1000m and 89mm at 2000m
Tiger I APCBC 102mm at 1000m and 86mm at 2000m
Sherman 76 APC 89mm at 1000m and 74mm at 2000m
Perhaps HTC has provided APC ammo for our Firefly.