You're rusty on your ochem, my friend
MW of epistane is 321.
At the 17th Carbon, oriented axially, there is an alcohol functional group -OH. During mass spectroscopy the -OH pretty much always leaves and takes a proton with it. That's -18 AMU from the total MW of 321. That would be the first thing to go, period.
That leaves 303 AMU remaining.
Sulfur, located directly underneath Oxygen on the periodic table (and therefore possessing very similar chemical and physical properties) is also likely to leave (as evidenced by PA's numbers himself, 288 being 321 - 33.) Epistane has an epithio group at 2a,3a. This means that carbons 2 and 3 are bonded to a single sulfur equitorially.
Sulfur has an MW of about 32 and it would strip a proton as well, making the total MW removed 33 AMU.
303 - 33 = 270 AMU.
To recap,
Epistane contains two functional groups that would likely leave under GC/MS conditions. On is an epithio group and the other is an alcohol. When the alcohol leaves, it takes a proton with it making the total MW loss from the parent molecule 18 AMU. The epithio group would strip a proton with it as well, that takes another 33 away.
So, methyl-epithiostanol = 321. - SH, -H2O = 270.
Dr.D is right on this one. I'm actually a bit appauled that PA would lie about it. An M+ of 288 makes absolutely no sense. He would have had to leave the -OH functional group on to get that weight, and -OH leaving and stripping a proton (coming off as H2O) is pretty much a given.