CTRL+U, CTRL+SHIFT+F, enter [345]+
Evo v3.30.5 (Debian Buster)
Also confirmed in v3.34.5 (Fedora 31) by Andre Klapper
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Enter CTRL+U on an email (View eMail Source)
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Enter CTRL+SHIFT+F (Search eMail Source)
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Enter "4" (Search for the numeral 4 in the eMail Source)
The numerals 3,4,5 don't work in the Source Search box, they do work in the normal email search box. Additionally, # (SHIFT+3), $ (SHITF+4), and % (SHIFT+5) do work in the Source Search box.