A detective story, not an answer
I pulled the historical TLEs for the ISS from Space-Track.org. On 16 May 2016, it experienced a rev rollover. The TLEs around the rollover have rev's 99,989, 99,992, 1, 6
I then pulled up the historical TLEs for Explorer 7 in the time frame you suggested around 21 Aug 2019 - 27 Aug 2019. It includes the two TLEs you pointed out. From the TLEs we know that mean motion is around 14.94/revs per day, and the TLEs are roughly daily. Yet rev numbers are basically oscillating between ~42,600 and ~18,025 as @uhoh points out in his answer. Of note, object number is listed as 22U (no leading zeros; keep reading)
At 14.94 revs/day, the rev counter should rollover every 18.3 years or so (closer to 19 on average due to orbit getting lower over the years). So I repeated the exercise around Dec 1978. Sure enough we see a rollover from 99,982 to 00123 in two consecutive TLEs (16 Dec and 26 Dec). Of note, object number is listed as 00022U (see extra leading zeros; keep reading)
Fast forward another ~19 years to Nov 1997. Another rollover! Orbit 99,986 to 00016 (Nov 2 and Nov 3)
I then tried looking around 2016 for the next rollover. Surprise!
Something funny happens when I pull up TLEs for 2015 and 2016. I get two sets in the report. The first set starts around June 2015 (even though I requested January) and goes through Dec 2016. In this set the object ID is given as 22U and rev numbers go from 19,562 to 28,124. They are increasing, but nowhere near the rollover. Although the span of 8000ish revs is about right for 18 months.
But then in that same request I get a SECOND set of TLEs, this time with object ID 00022 (extra leading zeros) and spanning the full period of 1 Jan 2015 through 31 Dec 2016. Here the revs go from 92,771 through 3,618, rolling over on 3 May 2016. About 18.5 years, as expected!
I then asked for TLEs from 2017-2019. All have object number 22U (no zeros). On 31 Dec 2017 we are at rev 33,597 which is consistent with the 28,124 from 31 Dec 2016 in the 22U data set (~5400 revs per year). On 7 Dec 2018 rev is 38,697 again consistent with ~5400 revs per year. But then on 8 Dec 2018, rev number jumps to 14,206!! Roughly consistent with a rollover on 3 May 2016 from the 00022U data set (and original launch).
The rev then starts oscillating between the ~38,700 number and the 14,200 number slowly increasing until it gets to the 42,600/18,025 range around August 2019, as you discovered. So the increase that @uhoh pointed out in the bipolar distribution seems to be consistent with ~5400 revs/day
Something happened in 2015 that caused JSPOC to issue slightly different formats - namely removing leading zeros from the object ID. The transition also caused the rev counter to jump to a seemingly incorrect number. Then around Dec 2018 rev numbers from both data sets seem to be included, roughly alternating every TLE. Both rev trends, however, are consistent with the ~5,400 revs per day.
In parting I leave you with this nugget from the AMSAT TLE description page:
"The folks who compute satellite orbits don’t tend to pay a great deal of attention to this number! At the time of this writing , elements from NASA have an incorrect Epoch Rev for Oscar-10 and Oscar-13. Unless you use the revolution number for your own bookkeeping purposes, you needn’t worry about the accuracy of Epoch Rev." Note: I don't think TLEs have EVER been issued by NASA, it probably should say NORAD.