This might be one of these stories where the designation "SVT38 mag" may just be a collectors title. Yes, the tabbed magazines are an early magazine variation. As a result, these tabbed magazines would have been used primarily on SVT38's and early SVT40's.
The question is whether they stopped the production of these tabbed magazines when they stopped the production of SVT38's?
Or did they simply stop the production of these magazines during early SVT40 production in a means to cut cost/reduce production time?
Unlike Mkb42(H), MP43, MP44 mags which have the designation on them, these magazines lack such markings.
As a result, this variation may simply be an earlier variation of this magazine, which would be correct for an SVT38. However, during service, I am sure that SVT40 mags were used in SVT38's.
There were several examples of where SVT parts changed in order to reduce production time/cost.