Wikipedia
Rüstsätze (Luftwaffe) were field modification kits produced for the German Luftwaffe during the Second World War. They were packaged in kit form, usually direct from the aircraft manufacturer, and allowed for field modifications of various German aircraft used in World War II, predominantly fighter bombers and night fighters. Rüstsätze kits could be fitted in the field, as opposed to Umrüst-Bausätze kits, which were typically fitted in the factory. This was not a hard and fast rule, however; during production runs various Rüstsätze kits were often fitted by factories in order to meet Luftwaffe demands, and "/R" designations were also occasionally applied to more complex changes in an aircraft's airframe design that were much more suitably completed at production line facilities, as with a few of the "/R"-designated versions of the He 177A-5 heavy bomber.