White Collops are (usually) slices of veal served in a white sauce.
The veal is sliced thinly and about as wide as a crown piece (2 inches / 5 1/2 cm.) The pieces are simmered in enough water to cover them, then taken out of the simmering water. The simmering water is used to help make a white sauce. The collops are added to the sauce along with seasonings, chopped sweetbreads, already-cooked mushrooms, and a few already cooked meat balls. Simmer for 5 minutes, then stir in egg yolks and cream. Just before serving, squeeze in juice of half a lemon.  Briggs, Richard.The English Art of Cookery, According to the Present Practice. G. G. J. and J. Robinson. 1788. Page 245
White Collops and Cucumbers
Cut small and thin collops from veal slices, and brown in a buttered pan. Rremove from heat, put them in some hot white sauce. Serve with braised cucumber. 
Rissolles served as White Collops
Rissolles served with a white sauce. The common element appears to be the white sauce. 
 Briggs, Richard.The English Art of Cookery, According to the Present Practice. G. G. J. and J. Robinson. 1788. Page 245.
 Nutt, Frederic. The Imperial and Royal Cook. London: Mathews and Leigh. 1809. page 82.
 Clermont, B. Rissolles Section in The Professed Cook: Or, The Modern Art Of Cookery, Pastry, And Confectionary. W. Davis. 1776.