Nail.

[Garabaldi]

Any ideas what this is and why it would have the number 51 on it?

 

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[daroofa]

Railroad date nail would be my guess.

 

[up north]

Could be a railroad date nail.

 

[turtlefoot13]

what daroofa and up north said

http://ny-image0.etsy.com/il_fullxfull.161932384.jpg

 

[Bigcypresshunter]

Date Nails were used for showing ownership, treatment purposes, identification, sequencing poles and bridge timbers. Dating ties and poles for replacement purposes. Some nails show the height of a pole, or the wood it is made from. This is just a few of the many uses for Date Nails.
http://www.nailhunter.com/

 

[ivan salis]

railroad tie or telephone pole nail -- both need to be replaced after a certain amount of time --thus their "dated" or marked --so you can keep track of em -date wize

 

[dick99344]

Railroad date nail would be my guess.

Yep. Here is one I took from an old tie of the Milwaukee Rail Road. 1935.

 

[Garabaldi]

First time I hear of of this. Thanks all.