i am begining to think the gun has a different model name in the US
based on the barrel steel and date of manufacture Wilsons Book (if you believe it) says 425 maybe these are called GR's just like the S50E sereies seemed to go by the BL name
FWIW from the book "The Model 425 succesor of the model 409PB A 3 lock gun with visible triple lock bolt"
the 424 reads Same as model 425 but with positive double bolt only