From Team Associated:
This optional, tall rear shock tower allows your RC10B64 and RC10B64D to accommodate longer, higher-capacity 31mm shocks for better compliance on very bumpy or loose tracks.
Associated has these optional 31mm shock bodies in two versions:
#91482 12×31 mm V2 Shock Bodies
#91578 FT 12×31 mm FOX® Shock Bodies with Genuine Kashima Coat, V2, threaded
#92013 – $24.99 MSRP