Q-RTF-X
I have a vague recollection that (removed pins) were normally kept in one of the rear crew desks, one of the Nav positions I think..
FJJP
If there ever were an occasion where the rear crew had to get out through the top on the ground, the Nav Rad had a number of pins which he could use to secure the ejection seats.
That explains it - I think. The 'main' pins were stowed upstairs - but I do have a foggy memory of a full set floating around downstairs. Nav Rad position would make sense.