The X43 series were 40' post-war steel box cars based on an AAR design.

With more than 42,000 cars built for the American railroads and 38,000 for the Canadian roads, the postwar AAR 40' boxcar was one of the most widely used freight cars. Many of these cars saw service into the 1980s. Like their 50' design, the AAR 40' box car was modular in concept and the primary aspects of the design standard were dimensions that must be conformed to. The side panels, ends, and roofs were nothing more than component parts that could be used interchangeably.

The 50 ton postwar AAR 40 foot boc car was on illustration of a standard car in a continuing series of AAR standard designs that began in 1932. THe original 1932 AAR had a standard inside height of 9'11" and the 1937 AAR standard box car had a 10'0" inside height. In October, 1947, the Committee on Car Construction revised the inside height from 10'0" to 10'6" because there had been little demand for the 10'0" height car. Standard features of the postwar AAR 40' box car were as follows:

10 foot riveted sides | 10'4" to 10'6" inside height | Improved Dreadnaught Ends

6', 7' or 8' door openings | Tabbed side sills | 50 ton capacity | 3836-3902 cu. ft. capacity

There were a number of construction variations and options available, but the 10 panel riveted side panel arrangement was by far the most common.

The postwar AAR box car was essentially the same as the 1937 AAR Modified Standard box car (sometimes referred as the 1942 AAR box car), but the 5/5 Dreadnaught End was replaced with the 4/4 Improved Dreadnaught End. This end design was introduced in 1945 and was extensively used on forty-foot AAR box cars through 1948. This was a period when the railroads were upgrading their fleet of rolling stock after World War II had ended. It was also used beyond 1948 for Santa Fe cars built through 1952 and Canadian cars built as late as 1957. The rib contour was the "rolling pin" shape and the ends came with a round corner and W-section internal bracing.

In 1948 the diagonal panel roof was introduced as well as the R+3/4 early version of the Improved Dreadnaught End. Thus, 1948 is considered a transition year for most American-built cars. In 1955 the Improved Dreadnaught End was further modified and this later version featured a banana taper rib design. The Murphy panel roof was commonly used on cars built from 1945 to early 1948. During the early part of 1948 the roof design changed and the diagonal panel roof became standard. The car builders had but one source, SREM, for buying ends. However, SREM also required the builder to buy their roof when purchasing ends. Doors and other appliances varied widely in application. Youngstown steel doors were used extensively, but Superior 7 panel doors also saw widespread use. Steel running boards were typical.

by Ed Hawkins, as included with Branchline kits

In HO scale, the X43B and X43C box car is available as a plastic kit from Atlas (formerly Branchline Trains). C&BT Shops offers kits for the complete series, however, the kits decorated for PRR Circle Keystone include incorrect doors and ends. Their undecorated kits may be used to accurately depict Pennsy cars,

Cars on the PRR Roster (as listed in the Official Railway Equipment Register (ORER)):

The X43

• Number series: 603500-603999, 500 cars
  Built by Greenville (order #549), Dec 1950
• 7' Youngstown doors
• 10 panel riveted car

• R+3/4 early improved dreadnaught end

The X43A

• Number series: 602000-6034999, 1,500 cars
  Built by ACF (lot 3476), Dec 1950-Feb 1951
• 7 foor Youngstown doors
• 7 rung ladders
• 12 panel wielded sides
• Diagonal panel roof
• R+3/4 early improved Dreadnaught ends

The X43B

• Number Series: 85400-86399, 1000 cars
  Built by Pressed Steel Corp., Feb or April 1951
• Number Series: 600000-601999, 2,000 cars
  Built by Pressed Steel Corp., Dec 1950-1951
• 7 foot 7 panel Superior doors
• 10 panel riveted sides
• 8 rung ladders
• Diagonal Panel Roof
• R+3/4 early improved Dreadnaught ends

The Branchline decorated kits have their frame and underbody details in black, but the color should be Freight Car Color (FCC) to match the prototype. The Branchline kit comes with AAR standard seven rung ladders but also includes correct eight rung ladders in PRR decorated kits, 

Andy Cich adds:

Branchline did two runs of the X43b cars. The first run consisted of car numbers 600538, 600635, 601254, and 601926. The 2nd run had car numbers 600532, 600633, 601185, and 601931.

The first run had the correct Superior doors. The second run had the incorrect Youngstown doors. So, Branchline does make the correct doors. An undecorated 7' door kit comes with both doors.

Atlas (who took over the Branchline line) does offer some Branchline parts. And it looks like the 7' Superior door is available.

https://shop.atlasrr.com/p-4063-ho-7-superior-doors-2.aspx

The X43C

• Number Series: 70400-71899, 1,500 cars
  Built by Pressed Steel Corp, (Mt. Vernon plant), Sept 1951
• 8 foot Youngstown doors
• 8 rung ladders 
• 10 panel riveted sides
• Diagonal panel roof
• R+3/4 early improved Dreadnaught ends

The Branchline decorated kits have their frame and underbody details in black, but the color should be Freight Car Color (FCC) to match the prototype. Branchline includes only 7 rung ladders, which matches PRR drawings, but photos clearly show 8 rung ladders as used on other subclasses.

Branchline Trains X43C model built by Jerry Britton