Before this Credor used 8L (lower grade version of GS Seiko 9S) and 4S (52 stream) movements. The 8L is very thick (~6 mm) and 4S is middle of the road (~4 mm). In 2005 they launched models with 4L (3.6 mm) movements--which were likely made from Soprod ebauches.
While it isn't very fancy it's slimness makes it the best fit for the Credor brand. Seiko started producing its own version in-house sometime before 2017, which is the 6L (2.7 mm). There is a long tradition of Credor movements being gilt instead of rhodanized, which explains the finish.