Fashionable Cavaliers of the 16th Century wouldn't dream of venturing forth in the chill spring breeze clad in anything save doublet, trunk hose and flowing cloak. Whether you dub it a doublet (now archaic), vest (the U.S. favorite), waistcoat (the British choice) or weskit (a dialectal variation of the latter), the short, snug, sleeveless, buttonable job worn beneath your 20th Century jacket carries on as an apparel item of singular distinction -- perfect for adding a dash of color to your town suits, or that necessary bit of extra warmth for ides-of-March country wear. The re-emergence of the fancy weskit a few years ago saw some pretty wild stuff masquerading as acceptable fashion. No longer so: yesteryear's elaborate brocades and flowered-wallpaper designs -- complete with platter-sized ornamental buttons -- are out as out can be. The news is clean conservatism with solid colors, quiet plaids and the traditional small checks like tattersall dominating the field. The four crafty clubmen taking their ease are sporting, from left to right, an all-wool tattersall weskit, $22.50, a Black Watch tartan vest with silver buttons, $19.50, a fire-engine red waistcoat with patch pockets, $12.50, and a four-pocket suede doublet with wool plaid back, $25.