Is the camera M or LTM?
It's likely a LTM lens produced from 1930-1959. They made over 300,000 of them it's VERY common.
The M mount lens was made from 1954-1961.
There's no apparent collectors value in having a 1951 lens Except in the mind of the seller.
I believe that 1951 was the issue of the red lettered 3.5 Elmar. THE sharpest of the 3.5 Elmars which were the lenses that made the Leica famous.... The 3.5 was produced till 1959, the 2.8 was sharper, the pre WWll 3.5's were uncoated and have there own following. The f2 Summar is another early lens with IT'S own following pre WWll and also uncoated, very smmooooth at wide open, especially with color. Very inexpensive, prone to scratches, fog and cleaning marks. "Clean" ones are findable, also a collapsable lens like the Elmar.
The real voyage of discovery consists not in seeking new landscapes, but in having new eyes.
My first Leica (in 1952) came with a new 3.5. The faster Nikkor and even a Minolta f/2.8 available then seemed to outperform it. I agree with Bill: the f/2.8 was sharper; one of the sharpest I ever tested.