Programming adapter to allow for higher Vpp programming voltages ZD1 should be approximately 3V lower than max Vpp – use lab power supply on 25V line.
https://www.dr-bosch.com/volker/umbau/index.html
D1 is BAT49 – but probably 1N4148 will work fine here, BC817 / BC807 probably can be any small transistors: 2N3904/2N3906