Remove the grips and try inserting the mag. Gunbugs is correct that grip screws can be too long if the wood has shrunk any in the last 80-100 years which can keep the mag from fully inserting. Also, have you tightened your grip screws in the last week or so?
If it still won't go in then remove the entire upper and try inserting the mag. If it won't go in using the above test but goes in after removing the upper then at least you know the problem is somewhere in the upper assembly.
-Bob
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