That is a Columbia Motobike frame with what is commonly refered to as a "coffin" tank. It is a post war bike that is shown in my 1948 catalog and was produced into mid to late fifties.
Initially it was offered (1948 for example) as the top of the line model and peaked in 1950 as the model N5 which was fully loaded with a chrome and painted tank, Air Rider Floating Action springer and Sturmey Archer front drum brake. 1951 is when the new style Columbia tank was introduced and became the top of the line model, reducing the role of the coffin tank models. By 1956 it was offered as a base model tank bike.
We are going to be publishing our post-war Columbia book some time in November and it will cover 1946 to 1959/60. There will be approximately 120 pages, with 20 being full color. All full page scans of original factory literature that illustrate and list all of the options on this and other Columbia post war models. It will also contain the Columbia serial number list so that you will be able to identify the year of your Columbia.