This "strap" is Ford part number FAA-18954-A and shown in the 65 and 66 Mustang Electrical Manuals (the 64 page is missing) for heater cable use. It is also used thru 1970 for various heater hoses applications. See the AMK 2014 catalog , p173, or AMK p/n B-13197 on their website. It's also referenced in this Forum's library in the Hardware Spreadsheets.
Jim