The 21 is a 47mm Special Edition Radiomir like the 249 and 309/322. Like the 249, it is a simple face with hours only and no seconds hand. The 21 has numbers where the 249 has no numbers (at 12, 3, 6, 9) and it has no numbers where the 249 has numbers. Like the 309/322 the 21 has a high quality third party movement (Rolex vs. Minerva), gold hands, arabic numbers, an alternate case material (platinum vs. ti), and an exhibition back. It has some qualities of both watches.