This page lists part number
53995 with the following description:
Reinforced Upper Arm Mount - TRF501X. Off-road tracks often have big ramps for long distance jumps. This new Option Part features a thicker shock-absorbing section and reinforcement rib to handle the impacts from hard landings and is only 0.7g heavier than the standard part.
http://www.tamiya.ca/DB-01/db01_hopup_listing.htm
What is that? Has anyone seen or used it?