Pocket watch mainspring replacement

[dnhb]

I've not bought a replacement mainspring before. According to my digital calipers the broken spring in my Lancashire Watch Co. Pocket Watch is 2.4mm wide & 0.2mm thick & the barrel is 16mm diameter.  The nearest Cousins has to this (GR6573) only differs in the barrel width - it's 15.5mm . Is this difference significant? Also, the old spring has a pin across the outer end to secure it in the barrel but the Cousins info implies it's "a normal bridle" (whatever that means) rather than a "T" (which I assume is how my old spring is categorised). Help, please!

[AndyGSi]

What's the full details of the movement and can you post photos of the old spring.

The 15.5mm shown on the GR6573 indicates the smallest size of barrel this will fit directly from the washer.