Both Scott R173b and R174a are horizontal pairs that are imperforate vertically between the stamps. I have yet to personally see an example of Scott R173a, which is a vertical pair imperforate horizontally. The only example I can find record of is from the 2006 Cunliffe Collection auction held by Spink.
In the global scheme of things these don’t catalogue for very much ($325 for R173b and $500 for R174a respectively), yet one doesn’t encounter them frequently. I’ve only seen them come to market a handful of times in the last 20 years. If they were “front of book” stamps, you likely could add a zero to their catalogue values… a perfect example of the difference that demand causes. There is still much more interest in postage issues than back of the book issues.
From Bart Rosenberg:
Dollar value sheets were of 128 stamps; 16 horizontal rows of 8 stamps each, per the Boston Book. So there are probably relatively few of these pairs to be found, since the likelihood is that few error sheets existed to begin with. And pairs would have had to be collected and saved as such. Using the Boston Book quantity of 4,299,000, that works out to about 33,585 sheets total. Did someone forget to roulette them, or was it simply some broken roulette wheels? We will never know.