Hi,
Usually 1937 S/42's have the inside side plate number (isp#) the same as the first two digits of a 4 digit serial number -- example 4428, would have 44.
If a three digit number it is usually the first digit -- example 118, would be 1.
Some are completely blank and some are even one digit lower, or one digit higher, but the usual is as mentioned above.
The 1938 S/42's are found usually found with either the first two digits or one digit higher. --- Example 4428 could be found with either 44 or 45 and be correct. Same with example 118, could be either 1 or 2, and be correct. Then there are the few totally blank examples mixed in also.
As you can see, in the 1937 and 1938 variations there are no set rules. They are a mixed up bunch. It is nice to have one that follows certain guidelines, but that is not always the case here. After 1938 almost always the inside plate number is one digit higher. Your sideplate sounds correct to me.
No idea what the 3 represents on you sideplate but believe it is as Johnny stated and nothing to do with the serial number. Sometimes you will also see an X.
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