Where can I get a sample packet for the 51C board (LOR example)? What about any pointers on the selection process and information on the MOS?

All the info you need should be on the Acquisition Support Center page here if you click on your component. Last year I had the pleasure of being the President of the 51C Reserve accession board.

https://asc.army.mil/web/career-development/military-nco/field51/

For an LOR, you're basically looking for a flowery stamp of approval from your leadership (that is, unless there are specific skills or such that a board is looking for). For a 51C, you can look at the example at https://asc.army.mil/web/wp-content/uploads/2014/06/O-5-Recommendation.pdf

If you read the details, it's not saying that you're a logistic genus .. it's saying that you are dependable, a good NCO, etc.

Additionally, there have been many previous posts about 51C reclassification on RallyPoint. Suggest you do a quick search for them for additional information*
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* Tip. If you go to Google and type in what you are searching for and add 'site:rallypoint.com' to the terms, it will restrict the results to this site.

ie., 51c reclass packet site:rallypoint.com