Thanks Latent.
The LSD I was referring to is an aftermarket Subaru type:
http://www.rallispec.com/dif_gls_rst1903.html
From what I was told, this unit will fit my TY756: RST-1903 "Ver.5/6 STI Type RA and S201"
While the other unit, would require machining: RST-1913 "standard factory front LSD on all MY06 and newer STI models"
I am new to Subarus, hence the confusion!