There are lots of mid-range Japanese manufactured bicycles from the early 1980s that fit the bill, such as the Bianchi Sport SX, Centurion Elite, Centurion LeMans RS, Fuji Del Ray, Fuji Royale II, Maruishi RX-7, Miyata 710, Nishiki International, Raleigh Competition, Shogun 600, Takara Tribute, etc. These bicycles typically used a double butted CrMo or CMn main triangle with 27" rims to hit the sub-25 lb mark at a reasonable price. However, be aware that many migrated to 700C during the mid-1980s, so there are specific years to find the desired combination.