Maccari is offering a tune kit for the HW50 'newer models' and the HW99. I e-mailed them and asked them what the difference between an old and a new HW50 is, and they said that the older 50s have rounded stocks, sometimes don't have safety catches and that they are lower in power. Mine has a safety catch, the fore-end but not the pistol grip is squared off and it does about 9 ft/lbs at the moment, so it sort of sits between their specifications. I am only really interested in whether the kit will fit in the piston/compression chamber!