There's a 105 Cassette that use the same sizing as MTB 11 speed, quite possibly this is the hg700 mentioned by @MisterTomTom
You're correct:
if you try and install a CS-HG700 11-34 cassette on a road hub you need the spacer (same size as 10 speed).
@Tijs started
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There's a 105 Cassette that use the same sizing as MTB 11 speed, quite possibly this is the hg700 mentioned by @MisterTomTom