Sound like one of these. Ask your LBS for a downtube schynbald.

Greave:
Also called schynbald or jamber. Plate defence for the leg from knee to ankle, initially pro*tecting only the front, but later the whole lower leg. Constructed of two plates hinged together and shaped to the contours of the mus*cle. Schynbald and jamber are used in connection with the earlier type.