BSI to Swisco

A quick learner says:
Hi, I'm looking to replace 2x BSI 1/2" Channel Block and Tackle Balance Non-Tilt. The digits on the block and tackle are 17 20.

The second/longer block and tackle has the code 29 20.

I believe one is 17 inches and one is 29 inches.

How does this translate into SWISCO parts?

Thanks.
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Emily from SWISCO responded:

Hi there!

It looks like you have either our Series 351 or Series 390 channel balance, depending on the depth of the bottom shoe. Our Series 351 comes attached with a 1" deep shoe, while the Series 390 comes with a 1-1/8" deep shoe so you'll need to measure as shown below. Based on your stamps, the 1720 balance's metal channel should measure as 18" and the 2920's metal channel should be 30". We recommend double checking this measurement to confirm. When determining length, you only want to measure the metal channel. You would also need the 1720 spring strength option for the 18" balance and the 2920 option for the 30" balance. 

Comparison of the S351 and S390 balance shoes.
Comparison of the S351 and S390 balance shoes.

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