Forgive me if the brain is addled after being away from the job for over 18 months (and I was working on a Firms not Residential Delivery for the past 4 years), but once all the mail is in the frame, would it not be easier to just then add the D2Ds (for the 18% you have decided to do that day) as you pull the mail out of each slot (or in the case of double slots just slap a set of D2Ds front and back of that mail, rather than having to try and put the D2Ds actually into the frame slots...?
Stands back and waits for the flaw in this to be pointed out....