Excelent scan job. It's a fully detailed service manual of this model.
Text excerpt from page 17 (click to view)
5 June 1992
16.1.4 2 Sided Original 2 Sided Copy Mode
Inversion original cycle is used for this mode. All the originals [A] are fed starting from the bottom original sheet. First, the front side of the originals are copied. The copied paper [B] is stacked in the duplex tray. The originals [C] are inverted and return to the original table. The original feed is then repeated. This time the reverse side of the originals are copied on the reverse side of the copy paper [D], which is fed from the duplex tray.