Hi Brian,
The 2 BIL's were built in four batches. The first batch were built in 1935 (2001-2010)
A further three batches of 2 BIL's were built. The second batch were built in 1936 (2011-2048), the third in 1937 (2049-2116), and a final batch in 1938(2117-2152)
These last three 2 BIL batches had detail differences to the first batch, including a larger Guards/Luggage area (more space for Hornby's motor bogie), one third class 8 seat compartment replaced with a 4-seat coupe, and a different type of electro-pneumate control gear.
The last batch of 2 BIL units numbered 2117-2152 differed to the first three batches of 2 BIL units in having the large Spencer Moulton self contained buffers instead of the original oblong shank buffers originaly fitted to the other units.
I don't know if any of the early built 2 BIL's had their original buffers converted to the Spencer Moulton type, I would imagine that someone on this thread should be able to tell you.
I like your idea of screw coupling your 2 BIL's together. That is how I couple my 4 COR to my 4 BUF, and when I get one of my Hornby 2 BIL's I was going to screw couple it to my model of a 6 PAN. I don't know if 2 BIL's and 6 PAN's could or did work together , but I have seen a photo of a 4 BUF coupled to a 5 BEL ( both staffed by Pullman ?)
Regards
Bazza