Mix flour, sugar, mustard, salt and cayenne in top of double boiler.
Add eggs and whisk until smooth. Add milk. Stir in vinegar, slowly (or it will curdle); mix well. Cook over hot water until thickened, stirring constantly. Add butter.
Soften gelatin in cold water; add to hot mixture. Stir until gelatin is completely dissolved. Add salmon.
Chill, stirring occasionally. When slightly thickened, fold in whipped cream. Turn into a 2 quart fish shaped mold rinsed with cold water.
Unmold on lettuce. Decorate with slices of pimento, olive, and thin slices of radish set in appropriate places for eyes and scales.