UX27 was a work horse postal card and was printed on cream, off-white, various shades of buff, light buff, dark buff, yellow, and canary. In 1916, due to the war, UX27C and UX27D were produced. Both used a grayish white card stock. The UX27C used the same stamp impression as the normal UX27 while the UX27D used a recut impression especially noticeable in the hair on the back of the head. Your card might be a UX27C but it is difficult to tell from a scan. If you think you have a UX27C then send it off for certification.
I have several copies of UX27C and they are all preprinted. Some are back only, some are front and back. UX27C was sold only in sheets for the convenience of printers.
I am partial to APEX for US postal stationery but then I am one of the APEX examiners. Many of the PF examiners are are also APEX examiners. The choice is yours. I don't know how they compare timewise. I usually have items back in two days.