My parts manuals show that the 261 used in the AD series (54-55.1) used an 18" 4-blade fan.
Check this page for the 54 info. Look at the bottom of group 1.064. The water pump had casting #3836208 (see group 1.069 farther down that same page) I don't know if that is a short or long shaft. This was the same for 55.1. The 261 fan blades have more pitch than the 235 fans.
The 55.2-57 commercial and utility w/ 6 cyl., and the 56-57 2-ton w/ 6 cyl. (could be 235 or 261) used a 20" 5-blade fan.
The 58 series 6 (2-ton) w/ 261 or 283 used a 19" 5-blade, except Dubl-Duti, which used the 20".
My references stop at 58 for the 261 applications. I don't know if any of this is relevant to a 59-62 261 used in an AD.
