Nice images Charlie .... where'd ya get those anyway? ;c)
Fact is I have yet to see any definitive information as to just who or what the Cooper Bros. are/were, but certainly they did get job lots of #70's from Jensen several times over a span of many years. An Educational materials supply house for the East Coast region perhaps, or ?
Whatever the actual history behind these engines, I would rate them as rather "Rare", but not exceedingly so, as they do show up from time to time, typically a few a year, on ebay. It is not uncommon to see them in well used and even rather abused condition, though nearly pristine examples also show up occasionally, but less often.
Another rebranded Jensen engine that has surfaced in a very small lot of #85G's out of Israel, that were tagged in Hebrew, though they also had the Jensen tag unconventionally mounted on the firebox. I think there were only 15 or so that were found, though there may well have been quite a few more made in that batch.