"Martyntje" isn't a rare variant, it's an old-fashioned transliteration. "Y" is "IJ" in cursive form, & was commonly used before Dutch orthography was simplified in the 1800s. "IJ" in Dutch is its own letter (pronounced like a very broad [
AI], like the vowel sound in "bright", but almost with a Southern drawl). So you might see in written form "
Martijn" or "
Martyn" & they would be seen as the same spelling, except that in the second the individual letters of "ij" are joined.