Set 20 is a "best of" of both sets 18 and 19... three drawings instead of dozens, at a much lower price:
McDonnell Douglas drawing 255BJ00050sht1: General Arrangement, Siamese Tank, Delta Wing Orbiter (8154X3231 pixels)
McDonnell Douglas drawing 256-20-0001: General Arrangement Model 256-20B Booster (also shows Model 255 orbiter), 6416X3263 pixels
Martin Marietta drawing 899A2010642: Booster Overall TPS Assembly, 11472X3253 pixels
Space Drawing set 20 can be downloaded here (http://up-ship.com/blog/?p=5057) for only $5.50.
McDonnell Douglas drawing 255BJ00050sht1: General Arrangement, Siamese Tank, Delta Wing Orbiter (8154X3231 pixels)
McDonnell Douglas drawing 256-20-0001: General Arrangement Model 256-20B Booster (also shows Model 255 orbiter), 6416X3263 pixels
Martin Marietta drawing 899A2010642: Booster Overall TPS Assembly, 11472X3253 pixels

Space Drawing set 20 can be downloaded here (http://up-ship.com/blog/?p=5057) for only $5.50.