We do, the major versions have tag releases like mm7, mm8, mm9, etc. as defined by the current build, and minor patch releases too like mm10p14 as new sequences come in.

https://hgdownload.cse.ucsc.edu/goldenpath/mm10/bigZips/

Example, say you have 5 sequences: CAT, ATC, ATCG, CGT, and ATATA.

One way of combining them up together to build a transcriptome is like this:

5 sequences:     ATATA
               CG-T  ATC
             ATCG CAT

  Reference: ATCGATATATC

ATCGATATATC isn’t the only solution to these sequences, but as you get more sequences to try and overlap, the more the uncertainty goes down