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Detent for the fore and after postion of the shift throws in the trans internally (to hold it into position), and by shimming the trans detent outwards via the brass shim (s), lessens the engagement of the detent ball to the trans internal shift linkage.
So actual shift shaft on the trans if just above the input shaft on the front of the trans, leads to the internal linkage of the trans top/back, and the detent on the back of the shaft just below the shifter box assembly of the trans where its detent applies pressure to shifter internal assembly bar track.