Is this a Series 351 Channel Balance?

Thanks for posting. This looks like either our S351-29 or S390-29. The only difference between these two balance Series is the 'depth' of the bottom nylon shoe. The depth is actually not the dimension you show in your picture of the shoe with a ruler; rather, it is the distance between the 'toe' and 'heel' of the shoe. The Series 351 bottom nylon shoe has a depth of 1", whereas the Series 390 bottom nylon shoe has a depth of 1-1/8". Compare with your old balances and see what you think matches best.

A stamp option of 2840 is for a window sash weight range of 23 through 28 lbs. Let us know if you have any questions. 

Sorry for the confusion. See the photos below. 

The first picture is of the nylon shoe attached to the Series 390. The Series 390 shoe has a depth of 1-1/8"; as you can see, this is the dimension running along the 'bottom' of the shoe.

The next picture is the shoe on the Series 351. As you can see, the depth for this shoe is 1". Hope this visual aid helps.