Zenith watches were imported into the U.S. by Edmond E. Robert. This apparently started around the 1909 date of the linked-to ad, as earlier ads for Zenith watches don't seem to have appeared in the Jewelers Circular - Weekly and Horological Review.

A small number of highly jeweled, high grade watches (having 17-23 pocket watch jewels) were built for the North American railroad market. These seem to have been mostly sold into Canada. Nevertheless, there is documentation that at least one U.S. railroad accepted Zenith watches into railroad time service for use as a railroad watch. The movements are Lever Set and have a watch adjustment to five positions, or in the case of the Extra RR 56 grade, six positions. The movements were available in a watch size of 16, with some grades also being available in 18-size.

The last of the series, the Extra RR 56 grade, were exported to Canada in 1956. NAWCC member Peter Kushnir, a former time service official for CP Rail, has stated that:

As the Canadian representative for Zenith, located in Hamilton, Ontario in the 1950s, he recommended that one of their calibers be modified to include all of the features needed to be accepted as a railroad watch for use in railroad time service.

One thousand of these were imported, five hundred bearing ordinary Canadian Dials and five hundred fitted with Marginal Minute Canadian dials.

The "56" in the grade name represents 1956, the year they were built.

These watches were supplied out of Canadian distribution for use on the Alaska Railroad.

N
o Zenith or jobber literature has come to light that describes or lists Zenith grades that were suitable for use as standard watches. Nor is there a serial number vs. grade or date list available. Nevertheless, the following information has been developed from reports of surviving examples.

Zenith Railroad Standard Watch Grades

Grade	Size	Jewels	Approximate Serial Number Neighborhood	Comment
Prima	18	17	1,644,8xx	No 16-size examples reported.
Prima	16	19	1,730,4xx	No 18-size examples reported.
Superior		19	2,389,2xx
Superior	18	21	1,644,8xx	18-size, 23-jewel Extra grade intermingled w/ this S/N
Superior	16	21	2,856,4xx
Extra RR 56	16	21	4,679,xxx & 4,732,xxx	No 18-size made.
Extra	18	23	1,644,8xx
Extra	16	23	1,671,xxx

With the exception of the Extra RR 56 grade, the number of reported examples is very small, sometimes only one watch. These reports are subject to errors in reporting or in recording of the data.