I do remember probably 8-9 years ago, in the hong kong public library ,a chinese military engineering magazine published once every 2 months.one article contain article about ir suppression exchaust duct very similiar to ah64 apache. Jt show the chinese engineer was quite familiar with this design,Surprisely the design never adapted to z10,more likely due to weight penalty,instead z10 opt for simple upward exchaust duct.
Looking at z21 exchaust duct,the upward duct more like a temporary solution.