I am trying to discover the meaning of the S's which make up the Lancastrian livery collar widely given to retainers of Henry IV and Henry V - my enquiries to both the Duchy of Lancaster and the College of Arms have produced no definitive explanation.

From Wikipedia (http://en.wikipedia.org/wiki/Livery_collar): "Many explanations are given of the origin of these letters, but none has as yet been established."

There is more information at the above link.

My current PhD thesis examines the function and meaning of the livery collar during the fifteenth century. There have been many meanings proposed, some more convincing than others. A.P. Purey-Cust's 'The Collar of SS, A History and Conjecture' (Leeds, 1910) is a helpful and informative read, but hard to get hold of. For a more up-to-date text have a look at Ian Mortimer's 'The Fears of Henry IV', Appendix 7 (pp. 384-7). This provides a neat summary of the various meanings of the esses. Personally I believe that the esses had a multiplicity of meanings; something which I'm sure would have appealed to the fifteenth-century mind. If you need any more information I'd be more than happy to help.