other manufacturers to try to fit in?
But the biggest sin was the Flashmatic mechanism to automate flash exposure. With the advent of automatic strobe lights this feature  was obsolete before the lens arrived on store shelves.

The GN principal is simple in concept--couple the focusing ring to the aperture ring thereby changing aperture setting as the lenses focuses on the subject. The Guide Number of the flash determines the aperture/distance relationship.
Because the scaling of the aperture calibrations could not be changed without a redesign of the camera metering system, the scaling of the distance adjustment had to be modified to match that of the aperture scale. This resulted in three undesirable (in my opinion) features:

• The sense of focusing for the GN lens is opposite that of all other RE Auto lenses. This is confusing when switching between the GN and other lenses.
  
• To match the scaling of the aperture setting, the normal linear helix was replaced with a precision non-linear cam. The GN loses precision for close focusing where, because of the reduced depth-of-field, critical focusing is most needed.
  
• The non-linear cam precludes a depth-of-field scale.
  

The 50mm f/1.4 GN lens on the left and the lens it replaced, the venerable RE Auto 58mm f/1.4 on the right. The GN has a slightly smaller diameter and is a little shorter, the result of shortening the focal length from 58mm to 50mm.

These images show the distance scales on the GN (left) and its predecessor, both set at a distance of 10 feet. Note the very different spacing of calibrations. Also note the GN scale is backward compared to the older lens.

A close-up of the GN's raison d'être: the Flashmatic scale. In use, the aperture scale and focusing are adjusted until the red dot on the sliding lock is opposite the Guide Number of the flash being used. The lock is then slid forward to couple the aperture to the focusing ring. Note: the Guide Numbers are for ASA 100 film only. Any other film speed requires consulting a table in the instruction booklet.

The f/1.4 GN lens was the only Topcor with multi-coating, designated by the red 'M' on the decorative ring. This is evident from the reduced reflections for the multi-coating. Also, the reflections are blue or magenta, depending on the angle, compared to the amber reflection from the old MgFl coating.

The front glass of the 50mm is much smaller than that of the 58mm. Topcon could have made the GN with a considerably smaller diameter but chose to keep the standard filter size of 62mm.

An f/1.8 GN was also produced but ended up packaged in the same housing as the faster f/1.4. The front optical element is much smaller and deeply recessed, but the same 62mm filter threads were used.