Check Num. field is not shown for custom payment instruments

I have already experienced this issue in the public sandbox.

To reproduce this issue:
1) Click "Administer ... CiviContribute ... Payment Instruments"

2) Click the "Add Payment Instrument" button

3) Fill in the label as "Check for Sisterhood" and choose an appropriate Financial Account

4) Click "Create New ... Contribution" and choose any financial type, any contribution amount. For "Paid By" choose "Check for Sisterhood"

At this point, the field for "Check Num." disappears,

I created a quick fix by editing the file /CRM/Contribute/Form/Contribution.php

I changed line number 657 to look like:
TRUE, array('onChange' => "")

FYI: I created this payment instrument because the checks for the Sisterhood are deposited into a different bank account than the checks for the main organization, each of which has a different general ledger id. Is this the best way to set this up?