"No. 4262 is one of nineteen Sally bombers listed in Report No. 68, "Life of Japanese Combat Airplanes," 20 March 1945, five of which had serial numbers that began with the numeral 4 and the rest with the numeral 6. These first digits of the serials numbers are code numerals, designed to expand the true serial numbers to four digits and also to indicate a particular production series or to identify a particular factory of origin. The production serial numbers for one series was apparently concocted by adding a base number of 4000 to the true serial numbers. The five examples in this report are 4262, 4318, 4370, 4436, and 4492. A second series is represented by numbers running from 6075 and includes 6081, 6082, 6087, 6100, 6205, 6233, 6235, 6250, 6270, 6323, 6341, 6519, and 6580.
The way to interpret these serial numbers is to strip off the base number and read what is left as a true serial number. No. 4262, then, becomes plane No. 262 or the 262nd aircraft of a certain batch of planes. Allied intelligence analysts listed either an actual date of assembly or an estimated date for all of the planes show in Report No. 68. For No. 4262 they had to make an estimate. It was "Mid Jan 42." The date of crash of No. 4262 was listed as January 1945 on Luzon in the Philippines. This gave No. 4262 a rather long combat life of 36 months. It might be that No. 4262 was war-weary and being operated as a transport. That would perhaps account for this long life.
The engine in question could be mistaken for a navy Kasei Model 11 because the type of engine with which the Sally Model II was equipped was the Type 100 1450-HP Engine (Ha-101), the army's equivalent to the navy's Kasei Model 11. The difference between the run of planes with the 4000 base numbers and those with the 6000 base numbers was in the propeller equipment. The 4000 planes had the Japanese version of the Hamilton Standard Props (designated Ha-Type), and those with the 6000 numbers had the French-designed Ratier propellers (designated Ra-Type). If you have recovered a propeller, you should have a Hamilton type, since your serial number is 4262. I can't say much more about Sally production because I have never conducted a detailed study of the production sequence and the applicable serial-numbers characteristics and ranges of this army bomber."
