Adding to what PowerandPassion wrote. What he describes is how the Airspeed Horsa drawings are and I assume it’s a standard engineering practice.

Bottom left of the drawing is a box listing modifications starting with Airspeeds own and then the numbered service modifications. Therefore the drawing you are looking at has all those modifications already included into it and acts as the production version of the drawing. If you manufacture the part as per the drawing you’re doing up to that “mod level”.

Modification drawings for the Horsa (and I assume other aircraft) only detail the work that has to be done to existing parts/components to bring them up to that modification spec. Once complete you’d find the data plate on the part/sub assembly and stamp it to add the modification number to show it had been incorporated into the aircraft.