Petter engine made for Onan - prototype ?

[Witte 100]

This is my first conversation on the site.  I am looking for historical information about Petter possibly making an experimental engine for Onan back in the late 70's / early 80's.   The engine I acquired is a flat 4 cylinder air cooled boxer design,  but it is a Diesel engine with a CAV rotary injection pump  ( date code March 78 ) and electric start.  The engine has many similarities of Petter parts being used.  The engine tag states the oil capacity in liters, and the serial number is "EXP 687"  ( experimental ? ).  Model P74N-DA024/X06.  If anybody has information to contribute it would be very welcomed.  hopefully there is a Petter expert out there.   Thank you