Here's my 2 cents on the USRA engine question: Ron is correct, though he has a typo in his text... (500-504 vs. 550-554)
The Class J-2 Mikes, numbered 550-554, were the only true, original USRA Light Mikados the Monon owned, even though they were not the only Mikes the road had delivered during the "great war". The class J-1 Mikes 525-529 were not USRA design, even though they may have been delivered under USRA control of the Monon. Also, the cylinder diameter (28" Diam. vs. 26 " for USRA locomotives) and driver wheelbase differences preclude them from being of strict USRA design. (In my opinion the "Monon-style" cabs alone disqualify all J-1's and J-4's from being true USRA copies anyway.)
The J-3 Mikes numbered 560-565 are what I think are commonly referred to as the USRA Heavy Mikado copies, (they were built in 1926, so they couldn't be original USRA engines) and their cylinder diameter X stroke, driver diameter, driver wheelbase, cab design, dome placement, etc. match USRA Heavy Mike specifications. Although steamlocomotive.com claims the Monon reverted to their own design for the J-3's. However, this website also muddies the water with a chart claiming 16 of the Monon's Mikes were heavy copies, which means someone out there considers both the J-3 and the J-4's to have been USRA Heavy Mikado copies. Mr. Hilton's Monon Route book locomotive roster lists only the J-2 Mikes being of USRA design, although the builder's photo of J-3 number 564 on page 168 sure looks like a USRA Heavy copy to me. (With the exception of the capped stack of course.)