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An explanation of the two types of slide for Dellorto PHF and PHM carburettors

PHF/PHM slides are split into two categories, early and late. Early carbs need an early type slide, later carbs can use either. The way to tell which type of carb you have is to remove the top and slide and look inside at the base where the slide sits. Early carbs have a round base

Later carbs have a round base but the top and bottom is cropped to give  flats on the side like this.

The slides therefore differ in the size of their idle speed ramp, Early slides have a small ramp, later slides have a larger ramp that is given clearance by the cut outs in the base of the later carb

like this

You can see here the slide on the right has a larger ramp and will only fit a later carb.

Early type slides are becoming hard to find now as production stopped some time ago but later type slides can be  machined down to fit early carbs by removing material from the inside of the ramp either in a lathe or carefully with a dremel or something similar.

The numbers on the base of the slide refer to the slide cutout and Pump ramp so for example a 60/1 has a 60 (6mm) cutaway , the smaller the cutaway the faster the airflow over the atomiser and the richer the mixture, and a No.1 (/1)  Accelerator pump ramp..The lower the number the lower the ramp starts for the pump in the groove on the slide so the earlier the pump operates.