勞力士，罕有，不銹鋼自動上弦鏈帶腕錶，配中心秒針、日曆顯示、24小時指針及象牙色”火車軌”錶盤，”Explorer II”，型號16550

Among collectors, the present watch with ivory-coloured dial is without doubt the most sought-after version of all reference 16550 watches. This ivory/cream dial is special both because of its very warm and eye-pleasing tone and because of the way the "Superlative Chronometer Officially Certified" is displayed. Also known as "rail dial" it is distinguished by the symmetric division of the two lines below the centre: "Superlative" and "Officially" to the left, clearly separated by a vertical space (thus "rail"), and "Chronometer" and "Certified" to the right.

While dials in white and black exist for this fine Explorer II reference, the "ivory rail dial" model was produced in a very small series only.

Ref. 16550 with dial versions in white, black and ivory are prominently described and illustrated in 100 Years of Rolex - 1908 to 2008 by Franca and Guido Mondani, pp. 184-187.