they look like 1970s sash metal windows to me !Very difficult to get clean wfp.When they are old they seem to leach dirt from the sliding mechanisms ,also because one is "stepped back"from the other this is another cause of problems.it is very difficult to get the brush up to the top of the glass but it still disturbs the dirt,on the "inner window" Just bad luck im afraid with picky customer,i do some of these but i dont think they even know or care what they are like! However discretion is the better part of valour so unless your not too bothered about losing it ,be careful. Explain that the only real way to be sure is to do them trad but of course that would cost more with cherry picker hire etc.