[TR] valve spring sets

[Randall]

TRF P/N RFK439 is a set of valve springs for TR2 through early TR4 with
triple exhaust springs as original.

There is a note on the Moss web site that they may supply sets with only 2
exhaust springs, even for P/N 831-000
http://mossmotors.com/Shop/MoreInfo.aspx?PlateDetailID=487265&LanguageID=1

I've never quite understood why these double sets can be used with the early
head; and the later double sets can not.

-- Randall