Keep in mind that many, if not most, of the "27X1-3/8" tires sold now and in the past are actually narrower than some current "27X1-1/4"- sized tires.
Big ones (actual 32mm on wide rim) in 1-1/4" size are Michelin World, Tour, Panaracer Pasela and Schwalbe Marathon.
Current 1-3/8" Kenda, Cheng and Tioga tires are all narrower than those!
David Snyder Riding on 1-1/4" Paselas for the first time here in Auburn, CA usa
I may be getting in on the tail of this, but does anyone know of a source for 27 X 1 3/8 tires. Tandems just ride better with more rubber on the gound; I've got an older Taylor with 27 inch wheels that could use changing. What I have now is an old Michelin World Tour.