Here’s some facts I obtained from 2 sources:
Model Railroader's Steam Locomotive Cyclopedia:
page 69 - The USRA 2-8-2 Light Mikado was first delivered in 1918, during WWI 625 were built and after the war 1266 copies made.
The same boiler was used on the USRA Heavy Pacific, EXCEPT the firebox (used on the light Pacific). 63" drivers were originally used.
page 73 - The USRA 2-8-2 Heavy Mikado used 63" drivers and the boiler was no longer than the light Mikes, but the diameter swelled from 86" to 96". 233 were built with 957 copies.
page 142 - The USRA 4-6-2 Light Pacific was delivered AFTER WWI, in 1919. They came with 73" drivers. 81 were built, not sure how many copies.
page 147 - The USRA 4-6-2 Heavy Pacific was also delivered in 1919 and all 20 went to Erie. The firebox was the same as the Heavy Mikado, but the boiler was that of the Light Mikado. Came with 79" drivers.
Mainline Modeler has some pretty nice drawings of both the USRA Light Mikado and the USRA Light Pacific in 3/16” scale. I measured them and then measured my Williams 2-8-2:
MM May 1985, USRA Light 2-8-2:
From back of boiler band (at front end of firebox) to front of smokebox – 29’-3”
From back of boiler band (at front end of firebox) to end of cab roof – 17’-9”
Major boiler diameter – 7’-8” (looks like drawing shows outer jacketing)
MM August 1985, USRA Light 4-6-2:
From back of boiler band (at front end of firebox) to front of smokebox – 29’-0”
From back of boiler band (at front end of firebox) to end of cab roof – 17’-9”
Major boiler diameter – 7’-8” (outer jacketing)
Williams/Samhongsa USRA 2-8-2:
From back of boiler band (at front end of firebox) to front of smokebox – 31’-3” (2 scale feet too long, or ½” in 1/48 scale)
From back of boiler band (at front end of firebox) to end of cab roof – 18’-3” (6 scale inches too long or 1/8” in 1/48 scale)
Major boiler diameter – 7’-8” (measured with digital caliper)
OPINION - Other than the Williams boiler being 1/2" too long, it looks pretty good to me when compared to the MM drawings.
Also, the USRA Mikado came first (1918) with the USRA Pacific following in 1919.