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The quartet you have listed under this opus number is not Op.5 No.1. Op.5 No.1 is the only one of Richter's six quartets which received a modern reprint. It was from Breitkopf and Hartel. Op.5 No.1 is in C Major. It is Richter's best known quartet. It also has been recorded several times. This quartet in G Major is some other work.

Yes, and no.
Grove online is not very helpful: 6 Quartetto’s, 2 vn, va, vc (London, 1768), ed. in DTB, xxvii, Jg.xv (1914), as op.5 (Paris, c1772)
but Musik in Geschichte und Gegenwart has a bit more information:
The first edition (London, Longman,1768), and the 1914 new edition in Denkmäler der Tonkunst in Bayern (as op.5) based on it has 6 quartets (C Bb A Eb G D), and only the first in C had a modern edition with parts.
The Merton Music scans are copied from the 1772 edition (Paris, La Chevardiere), the only first edition that mentions "op.5": Here the quartets are in a different order, and the quartet in C is substituted by one in g minor (G Bb A D g Eb).
All seven have been recently published by Amadeus (2007), they employ the order of the first edition & the DTB edition and add the g minor quartet as op.5/5b.
I guess that should be the way to edit the imslp entries - would require changing all the work pages of all 6 quartets except No.2 and 3 presently represented. Maybe one of the administrators would like to fix this - or I will do it if nobody else does. I already added notes explaining the above to all 6 work pages.

Add-On: You can find the Op.5 No 1 quartet as first of the "6 string quartets" without opus numbers (scan of the first edition)

This is now fixed. The opus number assignation was by a Paris publisher in 1778, so it's questionable.