Green’s Dictionary of Slang

an affectionate message, written on envelopes of love letters.

1945		D. Bolster Roll On My Twelve 90: What letters! [...] always covered in kisses and hearts and SWALK and HOLLAND.
2002		‘Hectic James’ HecticJames.com ‘Stuff’ 30 Sept. 🌐 These are shorthand acronyms used by lovers, such as SWALK for Sealed with A loving Kiss. Other examples given were: BURMA: Be undressed ready, my angel; EGYPT: Eager to grab your pretty tits; HOLLAND: Hope our love lasts and never dies; ITALY: I trust and love you; NORWICH: (K)nickers off ready when I come home; BOLTOP: Better on lips than on paper (of a kiss).