My take on the HO Brick 101. I've only seen a couple of photos of this mythical stuff and I'm sure the vacuformed plastic wasn't this perfectly uniform, but like I said, until I find this particular unicorn out in the wild, this will have to make due.
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If you are still looking for the brick pattern, here is a link to one of the many pages of brick textured sheet from the Plastruct website:
PLAIN & PATTERNED SHEETS
International Supplier of Architectural, Educational and Hobby Model Parts.
www.plastruct.com
Their website is awful, but they have page after page of vacuformed textured styrene sheet in many scales. It is most likely where it was originally sourced from by the OG model builders. We had tons of the stuff around the modelshop when I was there in the 90's and 00's.
Below are a few brick styles that seemed close to what you want, but there are many others if these dont work.
91601 (PS-87) maybe 1/87 scale? HO?
91604 (PS-90) smaller
91606 (PS-92) even smaller
91611 (PS-97) also smaller