I get the impression from the information at the link above that the man's collar "stood up" as a "Prussian" or "clerical" collar would do so-- not that the Stranger was dressed like a priest from head to foot.
Weird incident of the pencil-dropping Stranger who went to see Lusk earlier.
--------------- Von Konigswald: Jack the Ripper plays shuffleboard. -- Happy Birthday, Wanda June by Kurt Vonnegut, c.1970.
Did Emily Marsh, her husband and John Cormack see the ripper?
Also, the clothing the unidetified male was wearing was described as a clerical costume? Is costume the correct word use there? Or was it proper regalia?
The one thing that works against this premise is that Mr Lusk received the "Box of Toys" postcard a few days earlier, and it was addressed correctly. The handwriting seemed to be the same as the Dear Boss Lettter according to Lusk and some members of the Vigilence Comittee who saw both correspondences. But it would seem improbable that the "prussian collar man" was the same author of both, since he didnt have Lusks address.
The description is solely of the man's collar, not his outfit in general, Alan, and it simply states that he was wearing an overcoat with a 'Prussian or Clerical collar'. A Prussian collar is a style that later had a small vogue in the 1960s, when it became better known as a 'Nehru collar'; it is basically circular, without lapels, and can be worn folded over or upright (information from 'The Gentleman's Gazette' online).
In any case, there is nothing substantial to connect this person with the poster of the 'Dear Boss' letter, nor indeed to connect that letter with 'Jack the Ripper' himself (it's a racing certainty that he wrote none of the hundreds of letters received; the signature of the crimes is not that of the kind of serial killer who enjoys teasing the authorities, as a small number do).