Previously established units.

The division, by order, may provide for the unit operation of a pool or parts thereof that embrace a unit area established by a previous order of the division. Such order, in providing for the allocation of unit production, shall first treat the unit area previously established as a single tract, and the portion of the unit production allocated thereto shall then be allocated among the separately owned tracts included in such previously established unit area in the same proportions as those specified in the previous order.