Is there any rest v10 API to retrieve the contacts linked to an account in the sub-panel?

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Is there any rest v10 API to retrieve the contacts linked to an account in the sub-panel in v10 REST API version?

I have a custom MANY to MANY relationship between ACCOUNTS & CONTACTS modules and a custom field called "principal_contact_c"

I'm trying to pull the contact data from the sub-panel in ACCOUNTS, find the record that has "principal_contact_c = YES" and add it to a field in the main layout.

I then want the other records in the sub-panel set to "principal_contact_c = No"

I can do the setting & updating records bit, I just can't find the records