Hello and thank you for your question!
Your Singer is a classic model made during the centennial era of the company, c.1951. It appears that the "M.R." refers to its specific place (factory) of manufacture in Canada where these were made during the period, likely Saint John's.
As for a valuation... Since these machines were so popular and there were so many made, many have survived. This tends to keep their prices at a moderate level overall. That said, condition becomes a crucial factor in ascribing a particular value to any one of these survivors. Since you say that your machine is in excellent physical and mechanical condition, it will be seen in the higher end of the range for such machines. At present, your fine, ever-popular, mid-20th century Singer would have an estimated fair market value in the range of $200-$250.
So, you were right on the mark with your thinking!
Again, a great piece of history that's probably as good at what it was intended for now as it was nearly 70 years ago when it was made. Thank you for writing in about it!
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Thank you, again.