Yours is a standard Rega hole. A Schroeder Model 2 will fit, as will a unipivot Magnepan and a bunch of other tonearms, if you decide against the Hadcock.
You may be able to squeeze by with the Hadcock, depending on the headshell slots and the particular cartridge, but it would be better if you could slot the hole for a proper fit. There are SME type mounts around that can provide an attractive solution. The ADC ASB-1 sliding base is one, but it is a rare beast, and I don't know the hole diameter without looking it up. Anyway, if you happen to be in the US, I may be able to hook you up with a guy who can make a clone of it that matches the Hadcock shaft for a halfway decent price.
You may be able to squeeze by with the Hadcock, depending on the headshell slots and the particular cartridge, but it would be better if you could slot the hole for a proper fit. There are SME type mounts around that can provide an attractive solution. The ADC ASB-1 sliding base is one, but it is a rare beast, and I don't know the hole diameter without looking it up. Anyway, if you happen to be in the US, I may be able to hook you up with a guy who can make a clone of it that matches the Hadcock shaft for a halfway decent price.