Hi,
Yep, that looks like the "backlight" window from a TR4/4A/5/250 hardtop. /ubbthreads/images/graemlins/iagree.gif
By the way, the original hardtop consists of two major pieces : the cast aluminum rear frame with that window - aka the "backlight" - which is semi-permanently fixed at the rear of the cockpit, and the removable roof panel, which was originally aluminum (first 500 units) and later steel, and is fitted with it's own headliner.
In original Triumph usage "surrey" refers only to the soft vinyl section and its folding support frame, which were used in place of the rigid hardtop roof, i.e. could be fitted between the backlight and windshield frame. The surrey was handy when the weather turned nasty and the rigid roof panel had been left at home, since it's too large to store in the car. The surrey top and its frame were a separate accessory to the hardtop, were not included in hardtop kits.
Most of reproduction hardtops consist of fiberglass frame and roof, with a Lexan window.
A lot of racers removed the glass backlight window to swap in a slightly lighter weight Lexan version (Perspex in England, I think). The plastic windows were also easily drilled across the lower edge with some ventilation holes. Perhaps that window you've got was removed during this sort of swap.
That window is relatively hard to find and pricey. If an original part, I believe you will find the "Triplex" logo in the middle, about an inch up from the lower edge of the glass. You could probably auction off on eBay for decent money, but keep in mind that shipping will probably require a carefully made crate and will likely involve oversize package charges. (In the U.S., it might be best sent ground freight or by Greyhound bus.)
/ubbthreads/images/graemlins/cheers.gif