Omega Apnea Dubois Depraz module

I have recently discovered the Omega Apnea and as one who has rebuilt a lemania 1345 a number of times am intrigued as to the construction of the Depraz module and why an eta base was used with the depraz module and not the lemania 1340 but can find nothing . As an aside the Depraz modules I have looked at remind me somewhat of the lemania 5100 chrono bits . Flat pressed steel .