Great question. If I understand you correctly, you are asking about a reservist covered under 3919 who obtains a loan and then, after going into military service, wishes to use the interest rate protections of 3937 on the loan obtained before military service but while covered under 3937 as a reservist. In other words, the question is whether a loan obtained as a covered reservist (but before "military service") is covered under the maximum rate protections once the borrower enters military service.
When we look at the definition of "military service" (in 3911), it still doesn't include reservists (whose protections are separately located in 3917). As I'm sure you know, the rate protections of 3937 apply to an obligation obtained "before the servicemember enters military service". Therefore, it would seem to me that loans obtained as a covered reservist are not technically obtained during "military service" and, therefore, the maximum rate protections would apply.
That said, I haven't looked into this before (as this would be rare), so maybe someone like Andy Z can chime in here...
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Adam Witmer, CRCM
All statements are my opinion, not those of my employer, and should not be taken as legal advice.
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