The Gibson B25-12N 12 string was first introduced by Gibson in 1961. It was Gibson's first entry into the 12 string market. With the onset of the Folk Era of the early 1960’s, well known guitar builders started producing quality twelve string guitars. Glen appears to have started his 12 string career with a 12N, which stood for "natural finish".
Glen is captured in several TV appearances with this guitar in the early 60's:
Here is seen singing "Kentucky Means Paradise" around 1962.
He also appeared on several episodes of Star Route playing the Gibson 12.
The Gibson B25 made the front cover of Glen's album dedicated to the 12 string..."The Astounding 12 String Guitar of Glen Campbell" which was released in 1964.
The Astounding 12 String Guitar of Glen Campbell
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It appears that Glen lost interest in the B25 after a few years as other 12 strings, both acoustic and electric, took the B25's place.