You are probably long gone, but in case anyone else is searching on this issue, yes the shaft is now shorter on a SMEG A1-2 selector switch as well. The SMEG shaft extension is no longer available but you can find suitable extensions if you search.
I found one on ebay. “EGO 47.72018.000 SWITCH SHAFT SPINDLE KNOB SHAFT EXTENSION 18mm 28mm”
https://www.ebay.co.uk/itm/EGO-47-72…0/142343731671
I had to cut the extension shaft on this down to 10mm length that goes into the knob, so when fitted it provides 11mm of extension to the 31mm shaft of the new selector switch giving a total length of 42mm. The original length was 40mm so the knob will be only 2mm proud. I added a 2mm spacer between the selector switch and the cooker front panel to make the fit exact!
Without the shaft extension the knob is likely to fall off or break after a while.