while called the 'early' the main difference is a newly tooled fighting compartment superstructure with non-beveled edges. This is the type that had to be ground away from in front of the side periscopes. To make an early, use the early exhausts (WC23 & WC24) and don't add the ventilators (WY3). Neil Stokes covers this and more here:
http://www.4bogreen.com/kv-series/recipes/su-152/su-152-mar-to-aug-1943
Regards,
Saúl García