How to understand US military aircraft designation on the vertical stabilizer?
The number on the vertical tail is the aircraft serial number, not the type designation. Here the serial number is AF 41 823. The '4' is the last digit of the finacial year in which money was allocated to manufacture the aircraft and the remaining 4 digits represent the sequence number indicating the sequential order in which the particular aircraft was ordered within that particular financial year.

From USAF serials:


  Starting on July 1, 1921 (the beginning of FY 1922) a new serial number system was adopted based on procurement within each Fiscal Year. Each serial number now consisted of a base number corresponding to the last two digits of the FY in which money was allocated to manufacture the aircraft, and a sequence number indicating the sequential order in which the particular aircraft was ordered within that particular FY. For example, airplane 22-1 was the first aircraft ordered in FY 1922, 23-1 was the first example ordered in FY 1923, etc. This system is still in use today. 


Apparently, USAF decided to have five digits in its tail number in 1958. This resulted in the cutting of first digit of the financial year.


  Camouflage began to reappear on USAF aircraft during the Vietnam War, and this led to a change in tail number presentation. The letters "AF" were added directly above the last two digits of the fiscal year, followed by the last three digits of the sequence number. The three-digit sequence number has a height of the AF and fiscal year letters combined and is sometimes called the "large" component of the tail number. For example, F-4E serial number 67-0288 had the tail number 67(small) 288 (large). This could of course lead to confusion, since aircraft 67-1288, 67-2288, etc would have exactly the same tail numbers as 67-0288 under this scheme. This would not ordinary cause a whole lot of difficulty unless of course some of these larger serial numbers also happened to be F-4Es (which they were not). Unfortunately, the system was not always consistent--for example F-4D serial number 66-0234 had a tail number that looks like this: 60(small) 234(large).


So, '4' represents the second digit of the finacial year, and 1823 gives the sequential number. The aircraft appears to be a F-100 Super Sabre. From f-100.org, the actual serial number appears to be 54-1862, a F-100C-20-NA model.