Its hard to know for sure what else was on the cover with this pair, if anything. The 3c rate certainly was the most common, but the 2c rate would have paid a circular to western europe. Also of course, this pair could have been on a cover in addition to some other stamps of a different denomination, to make up some entirely different rate.
You can always monitor
ebay and maybe 23L with a matching cancel will show up someday. I had that happen to me, with a pair that I used to have.