http://images.flightstore.co.uk/imag...9-08429600.jpg
Looks to me that the transponder control is the small box just above the blank panel near the bottom on the right hand side. In which case, it's a case of dial each number in - marginally more time consuming. The range of numbers available for each digit is 0-7 (it's an octal numbering system). Something like this:-
Best guess on that unit - turn the bottom left knob to STBY, use the big middle knob to dial in the four digits, then back to XPDR, or more likely to the TA/RA (Traffic Advisory Resolution Advisory) setting which integrates it with TCAS (Terminal Collision Avoidance System).
Control units vary a little - here's a picture of the Garmin box in the aeroplane I was flying yesterday..
That one, you just press the four numbers. But that's a GA unit, airliners tend to be more integrated and we don't fly with TCAS in most baby aeroplanes, whilst it's mandatory in airliners.
G