I don’t have any records of the serial numbers for these old mixers so that’s not a great deal of help, but relying on memory, as I recall all the 828S mixers were Mk2.
Mk 1 was a type with discrete transistors in the EQ and a very simple mic amp. Mk 2 was an improved EQ and a superior mic amp.
All the mono channels always had individual outputs, but the stereo channels did not… they had jack line inputs and also RIAA phono inputs.
Yes, the mono channels can take full line level via the jack line input.
The date of these 828S mixers was between 1979 and 1983.
That version that you mention is/was on ebay and was likely to be a little commercial production mixer for one of the local radio stations.