Given the similarities but minute variations of the superb Vltor RE-A5 and RE10/A5SR, we have taken some comparison pictures to make it easy to distinguish one from another, also to illustrate the functional differences.
Vltor's A5 requires a longer A5 length receiver extension (RE) to accommodate the A5/rifle length spring and longer A5 buffer. RE-A5 and RE10/A5SR are of the same length, whereas RE-A5 has 7 stock positions, RE10/A5SR has 6.
The extra position on the RE-A5 provides slightly more adjustment, but more importantly, it allows longer stocks like the Magpul STR (7" in length) to more fully collapse than the RE10/A5SR, as shown below.
Note that this is only applicable to longer stocks. The 7 position vs. 6 position difference is irrelevant on stocks with lengths that are close to the Colt M4 stock, such as the SOPMOD stock, which is about 6.5".
Another difference is the RE-A5 has more threads for the castle nut than RE10/A5SR, but this has no impact on functionality.
It is probably not news we're a huge proponent for the Vltor A5 system. We're a fan of the company, the wonderful folks that run it, and its engineering prowess.
The Vltor A5 is a well engineered, tested, and vetted system that brings real benefits to the reliability and functionality of the AR platform. Its elegant simplicity, reliability and benefits make the A5 one of the most worthy and best innovations for the AR15/M16, it transforms a rather violent carbine into a smooth shooting machine, which has direct correlations to the longevity of a carbine's components the short carbine gas systems beats on mercilessly.
For a more detailed look into the Vltor A5, please visit this page: https://www.arbuildjunkie.com/vltor-a5-buffer-system-overview/ and https://www.m4carbine.net/showthread.php?136025-What-s-so-great-about-the-Vltor-A5