Sorry, I can't corroborate the diameter of the seat post used on the International LD based on personal experience. However, while the frame uses large diameter tubes, the seat post is smaller than you would think, as the the tubes are joined using internal lugs. So, 27.2mm could be correct. I have no reason to doubt this figure given that it is stated in a 1986 road test and the 1987 catalogue. If you have a copy of the 1986 catalogue, I'd appreciate be able to see it.

I'm assuming that you have or are getting/considering a frame with no seat post. If you have calipers, try measuring the inside diameter of the seat tube lug. The opening is often slightly distorted by up to a few tenths of a millimetre, however it should tell you if 27.2mm is reasonable. While even less accurate, careful measurement using a ruler should get you within 0.5mm. Alternately, you can try various size seat posts or take it to an LBS where they can use a gauge, vernier or test fit various size posts.

Before you measure, ensure the cinch slot hasn't been pinched at the top, as the measurement will be smaller than the true diameter. If it has been pinched, gently pry it open with a wide, flat bladed screwdriver or similar tool, until the sides of the pinch slot are parallel from bottom to top.