Channel Balance Top and bottom identification

A home owner from Maryland says:
March 12, 2017
The channel balance is 28-5/5" long, width 9/16", depth 11/16". The sash weight is 21.2 lb.

The balance is marked 28 C along it's length and BSI across as in the picture.

Please advise what channel balance is suitable for these windows. Also, please indicate what the top and bottoms are.

The windows are non tilt approx. 36" x 64". There are no markings on the window itself, but will you be able to know the manufacturer from the channel balance marking?

Thanks,
Satish
A home owner from Maryland says:
March 13, 2017
The previous channel balance bottom was broken. Here is a new picture.
Channel balance
Profile picture of Paul
Paul from SWISCO responded:
March 21, 2017
Hello Satish,
It looks like you have a Rollyson window balance Series 371. The length of the channel should be 29". Please compare. If so you can use S371-29 and the spring weight option for 28 C is our stamp option 2830.
29" Series 371 Channel Balance
29" Series 371 Channel Balance
S371-29
$11.10
Top Sash Guide Pair, Rollyson
Top Sash Guide Pair, Rollyson
18-124P
$3.92
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