Since this question comes up a lot I thought I would repost my repost of a John post, assisted by Tim on the graph.

In December 1995, a series of Great Lakes lighthouses were introduced as a 'Stamp Set', matching the five lighthouses featured on a set of US postage stamps. Two of these five had already been done as limited editions (Marblehead OH and Split Rock MN) - but these were about to retire.

Harbour Lights decided to make all five of the 'Stamp Series' lighthouses open editions. 

The 1996 introductions then in production would include both open and limited editions -- and some of the open editions were lighthouses that had not been previously issued as limited editions.

Collectors began to complain - "Harbour Lights should not issue any lighthouse as an 'open edition' that had not previously been issued as a retired edition and retired."

The compaints grew and Harbour Lights listened. They anounced that the collectors' demands would become the rule. GLOW open edition lighthouses would only include those items that had been limited editions and retired.

That decision would be retroactive. It would cover the three Stamp Series lighthouses issued in 1995 and the 4 1996 GLOW lighthouses that had already been released, plus 2 that were already in production but not yet shipped (Peggy's Cove and Fire Island - which do NOT have edition sizes on their flags but were never issued with a "4XX" product number.)

The old collectors were happy - but new collectors were confused. They were confused because they were seeing lighthouses without edition sizes on the flags and some with serial numbers that began with letters, and eventually what looked like the same lighthouse with different product numbers.

And they would eventually be confused because Harbour Lights chose to re-use the 400 series product numbers that were originally assigned to the GLOW-turned-limited edition lighthouses.

So 410 (Spectacle Reef) became Cape Neddick GLOW
411 (St. Joseph) became St. Augustine GLOW
405 (Cape Lookout) became Ponce de Leon GLOW
406 (Fire Island) became New London Ledge GLOW
407 (Alcatraz) became Holland GLOW
417 (Mukilteo) became Alcatraz GLOW