bolts !
Two sizes of bolts are used in the 1-11 window ( 8-32 and 6-32 ) if I remember corectly and these bolts are quite similar if looked at under normal conditions.
The guy who installed the window was working outside at night and the only ilumination was probably his flashlight.
The bolts that were issued to him ( at BHX ) came from the engineering stores at LHR and had the correct paperwork with them.
The whole of the engineering workforce at LHR was on strike at the time over a 12 hour shift issue so the bolts were issued by members of management working in the stores.
The managenent stance to the 12 hour shift issue did an about face the day after the inccident and the LHR strike was resolved at once.
The CAA took no action aganst the engineer who fitted the window and BA moved him to a managenent position.
I will leave you to speculate how the wrong size bolts got into the window.