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We use 5 test data series to quantify putative discontinuities around 1946 in5 annual-mean sunspot number or group number sequences. The series tested are:the original and new versions of the Wolf/Zurich/International sunspot numbercomposite [$R_{ISNv1}$ and $R_{ISNv2}$] ; the corrected version of $R_{ISNv1}$[$R_C$]; the backbone group number [$R_{BB}$]; and the group number composite[$R_{UEA}$]. The test data are: the group number $N_G$ and total sunspot area$A_G$ from the RGO photoheliographic data; the CaK index from re-analysis ofMWO CaII K spectroheliograms; the group number from the MWO sunspot drawings,$N_{MWO}$; and ionospheric critical frequencies at Slough [$foF2$]. The testdata all vary with sunspot numbers, in some cases non-linearly. Tests use bothbefore-and-after fit-residual comparison and correlation methods, applied tointervals iterated to minimise errors and eliminate the effect of thediscontinuity date. It is not assumed that the correction required is by aconstant factor, nor even linear in sunspot number. A non-linear correction isrequired by $R_C$, $R_{BB}$ and $R_{ISNv1}$, but not by $R_{ISNv2}$ or$R_{UEA}$. The test datasets give very similar results in all cases. Bymultiplying the probability distribution functions together we obtain theoptimum correction for each data series that must be applied topre-discontinuity data to make them consistent with the post-discontinuitydata. It is shown that, on average, values for 1932-1943 are too small(relative to later values) by about 12.3% for $R_{ISNv1}$ but are too large for$R_{ISNv2}$ and $R_{BB}$ by 3.8% and 5.2%. The correction applied to generate$R_C$ from $R_{ISNv1}$ reduces this average factor to 0.5% but does not removethe non-linear variation, and other errors remain uncorrected. A test isprovided by $R_{UEA}$, which is identical to the RGO $N_G$ values over theinterval used.

Tests of sunspot number sequences: 4. Discontinuities around 1946 in various sunspot number and sunspot group number reconstructions