the catch with the original pultec output transformer is that it has a tertiary winding for feedback. most output transformers do not have a tertiary winding. that's one of the reasons the gyraf pultec has a different output amp design (also no interstage).
i dont want to discourage anyone from buying the sowter 9530, because obviously they have gone to a great deal of trouble to get a very close match with the s-217-d. however, another option, if you aren't stuck on an exact match, is the cinemag CM-9600-T. it is also 15k:600, with a 26.5:1 tertiary winding, and i bet you could get it made as 25:1, like the s-217-d. the CM-9600-T is the output transformer for the groove tubes vipre, and it costs $100 in small quantities (considerably more than the sowter). just another option.
ed