Hi, I'm a Welsh speaker who was fluent throughout school, and I'm now coming back to improve my language. 'Mas' - synonymous with 'allan', I don't know if this is taught - is not spelt with an apostrophe. I'm not sure why there is one here. Thanks!
It's probably there because the word is derived from maes, to mark the elided vowel.
The Gweiadur dictionary gives three spellings: maes ma’s mas (so both with and without apostrophe. The GPC dictionary, on the other hand, has an entry mas (only without apostrophe) that points to maes, though the historical citations use a variety of spellings (maes mâs ’ma’s mas).
Oh, right, okay! I understand now why the apostrophe is used. Although, I do think that it isn't really needed here, as in modern Welsh literature it's usually spelt simply 'mas'.
It's spelt "ma's" in the south version of the cwrs Mynediad books (and not used at all in the northern books).
Yep, as pointed out above, it's a remnant of its origins but there's no real need to spell it with an apostrophe these days.
They mean the same thing -- depending on where you live, you would usually use one or the other. Kind of like "pavement/sidewalk" or "torch/flashlight" or "tap/faucet" in English.
ma's is "more south" and allan is "more north", in case you want to pick your preferred word based on that.
And just to add to what @mizinamo says, as well as the above, allan is also the "official" version of the word too i.e. the one you'll see all over Wales on road signs, public notices, fire exits and so on.