PART NO.  SPL.PAGE  DESCRIPTION                                                           
AHH5506   15.N.8    Strap -horn harness (optional for dual horns)
AHH5506   15.Q.1    Strap for tail lamp cable to rear cross-member (2)
AHH5506   16.N.15   Strap -horn harness (2) -(optional dual horns, cables and connections,
    1600 and 1600-MK-II)
AHH5506   TC.N.10   Strap -horn harness (2) -(flasher-horn controls, electrical, Twin Cam)

There is another tech page on this web site grouping all "Clips and Clamps" for the MGA all on one page. That page has a picture with this part number showing it as a thin steel strap. I have thought for many years that this should be a rubber strap (considering the production era) to avoid sharp edge abrasion on the wiring harness. This strap is not well documented in factory literature. These days what you see or what you get may depend on which parts supplier's web page you might be looking at on any given day. Any supplier may be providing whatever they can find that may serve the purpose. If it was on my daily driver MGA in the 21srt century, it would be a plastic tie wrap. For a concours show car it might be either steel or rubber (not plastic). Considering that I don't know the original configuration, I suspect there will be no concours judge anywhere who will know for sure, so you may have grounds to complain if they deduct a point for this issue.

More informatiion on Dual horn Installation for MGA.
This strap may also be used to secure the rear wiring harness.