Unique '70's ETA2824 probably for chronograph attachment module

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I came across this early version (1970's?) ETA2824 and found it to be rather curious. It apparently must be for a chronograph attachment module perhaps a Dubois Depraz. It has a raised up cup type of calendar ring to accommodate an insert. The calendar drive wheel is special to accommodate this raised cup calendar ring.  It has a pawl and spring that must be for the attachment and a high post type seconds wheel that probably drove the chronograph. It also has a unique stem, not the twisted square type typical of '70's 2824's. I had a time trying to source a stem and I found something called a Rhonda 2013, but it didn't match the sliding pinion so I bought the sliding pinion that was also in the lot with the stem and that worked. I also had to add an automatic winding bridge as well as strip out the extraneous and unique parts and replace them with standard parts. A lot of effort to save a scraped 2824 but I had a feeling that this movement was originally intended to be for something special.