Mark Mathu wrote:
"What features make Walthers' HO 1944 AAR boxcar an AAR "modified" boxcar?  The 10'-6" inside height?"

Here is a typical model:
https://www.walthers.com/40-aar-1944-boxcar-ready-to-run-spokane-portland-seattle-11124 

You're confusing the terms.  "Modified" is used to indicate the 10 ft 6 in inside height versions of the 1937 AAR boxcar design as the first iteration of the design was 10 ft inside height.  It's not used to describe any variation of the1944 AAR boxcar design.

Walthers uses it correctly in the sales e-mail they sent out today.

Ben Hom