First, let me start by saying that the search function works, and works well, on groups.io. I've successfully done some homework in the archives before asking this question!

Micro-Trains has announced pre-orders for an N Scale Southern Pacific "impact car". This boxcar, SP 200, was re-purposed to demonstrate the impact of coupling speeds on lading.

https://www.micro-trains.com/index.php?route=product/product&product_id=2933

It is the same idea as the Union Pacific "impact car" which is well documented, including online at Don Strack's "Utah Rails" site:

http://utahrails.net/up/up-freight-cars.php then scroll down to UP 195220.

However, I've not been able to locate anything about the SP impact car online or in my own personal research accumulation. It's not in any of the three Color Guides from Morning Sun.

I suspect the answer lies in Tony Thompson's book Southern Pacific Freight Cars Volume 4: Box Cars -- and don't I wish I could own every book ever published on the subject of freight cars...

The most important data point I'm looking for is when this car was placed into service. I'm guessing that it was the early 1950s. (The UP car's time period is from 1952 to 1963.) <snip>