What aircraft would have these tail numbers?
I'm convinced this numbers were fligtradar-internal identifiers, nothing concerning the flights themselves. The following is not an answer, just what I know about flightradar data records and where to find data like yours.

The flightradar website on a computer updates its data by requesting this url, which contains coordinate bounds, filter settings and so on:

https://data-live.flightradar24.com/zones/fcgi/feed.js?bounds=40.19,39.78,-76.26,-74.03&amp;faa=1&amp;mlat=1&amp;flarm=1&amp;adsb=1&amp;gnd=1&amp;air=1&amp;vehicles=1&amp;estimated=1&amp;maxage=7200&amp;gliders=1&amp;stats=1

I guess the mobile app is not different in this point. 
The answer from the server is a JSON object like this:

{"full_count":14127,"version":4,
"b2fd160":["AD3C6B",39.9674,-75.9941,230,34000,368,"1306","T-MLAT2","MD88","N952DL",1475422448,"BDL","ATL","DL2457",0,0,"DAL2457",0],
"b2fe181":["A9FF54",39.9429,-75.9208,273,16860,378,"2347","T-MLAT2","B737","N743SW",1475422445,"PHL","PHX","WN601",0,1600,"SWA601",0],
"b2fc0bf":["",40.0910,-75.8714,230,39999,368,"1361","T-MLAT2","GLID","",1475422448,"","","",0,0,"GLID",0],
"b2fb3da":["A1A8DA",40.0949,-75.8623,98,8700,304,"3553","F-KPNE1","A320","N206FR",1475422446,"MKE","PHL","F91052",0,-896,"FFT1052",0]
...,
"stats":{"total":{"ads-":8655,"mlat":2916,"faa":1647,"flarm":346,"estimated":456},"visible":{"ads-b":6,"mlat":11,"faa":3,"flarm":0,"estimated":0}}
}


There is one line per aircraft, starting with a 7-digit, lowercase hex number, followed by any information about the flight in square brackets. (You can easily identify position, course, speed, registration, aircraft type and so on.)
From the structure of the file and from the fact that you can observe many subsequent hex numbers when you collect lots of data, it seems the first number is a flightradar-internal identifier, acting as counter. Whenever a new flight is detected, it gets the next number.

Within the square brackets, the first number seems to be the unique 6-digit hexadecimal (=24-bit) aircraft ID @JScarry mentioned (and the third entry, a glider, doesn't have one). But more interesting is that all data here is uppercase. (And according to this site, text messages from the aircraft like flight ID doesn't have upper/lower case)

Now, you have two subsequent, lowercase 7-digit hex numbers. This really looks like the flightradar-internal ID. So, maybe flighradar had a glitch, or it had absolutely no further information about that flights, and displayed the internal ID instead. What doesn't fit is the range, right now the numbers are about 164.000 (decimal) ahead. (EDIT: It can fit, they record 140.000-175.000 flights per day)

One can also retrieve data about flights with a certain internal ID from flightradar, but with your numbers, I get an empty record.

Yet, I think it was an internal ID, and you won't get the answer outside of flightradar.