The driver has an unusual rotating hatch which has vision blocks when rotated forward. Included with the set was an extra layer of steel belly armor which was bolted onto the vehicle's bottom, although only covering from the front to half way to the end, possibly due to weight reasons. A standard modification made during the mid-70s was the addition of the "Cereal Bowl" commander's cupola. This mod came about due to the "broken rib" effect that occurred when the Sheridan fired conventional rounds; the recoil would pitch the commander against the armor plating, resulting in cracked ribs. Post–Vietnam War service The Army began to phase out the Sheridan in 1978, although at the time there was no real replacement. Nevertheless, the 82d Airborne Division was able to keep them until 1996. The Sheridan was the only "rapidly" air-deployable tank in the inventory. Their units were later upgraded to the M551A1 TTS model, including a thermal sighting system for the commander and gunner. In the early 1980s, the M551A1 was fitted with a visual modification kit to resemble Warsaw Pact vehicles from Soviet BMP-1, ZSU-23, T-55 to T-80s, at the National Training Center at Fort Irwin, California. These modified vehicles were used in part of the U.S. Army's Soviet opposition forces (OPFOR) by providing realistic ground training to U.S. military units about Soviet combat doctrine in a desert environment. The OPFOR Sheridans no longer fulfill that role, having been retired at the end of 2003 and subsequently scrapped or made available as "hard targets" or, in a few cases, as museum pieces. Many were dumped to create artificial reefs. The Sheridan's only air drop in combat occurred during the United States invasion of Panama (Operation Just Cause) in 1989, when 14 M551s were deployed with C Company, 3/73rd Armor, 82nd Airborne Division. Four M551s were secretly delivered to Howard Air Force Base in Panama aboard a C-5 Galaxy in December 1989, days before the invasion. They were attached to TF Bayonet (193rd Infantry Brigade), and attached down further to TF Gator. These Sheridans took part in the attack on the Comandancia, initially supported by fire from Quarry Heights, and later displacing forward into the city. As part of Team Armor, these Sheridans later provided support to JSOC elements as they secured high-value targets throughout Panama City. The remaining 10 Sheridans were delivered via C-141 low-velocity airdrop (LVAD) to Torrijos-Tocumen Airport