Timely post for me as I am working on a very similar Raleigh International. Mine is a 74 in the same size but in much worse shape. I have a Brooks Professional that I have had for probably 40 years. I thought, “no problem” and put it on. No good in my situation and a bit of a surprise. It seems I couldn’t get it to slide/adjust quite far enough to the rear for my usual riding position. That was odd, as I don’t like my saddles too far reward as the only time I have had knee pain was with a seat slightly too far rearward.
My conclusion was that the frame rails on this 40 year old saddle was biased towards a more forward and aggressive riding position. Kinda odd, as I certainly don’t like a rearward position, and the International doesn’t have a radically upright seat tube. A B17 did dial things in just right, but I was a bit disappointed as the Pro would’ve been a plus on this old beater. I also concur with others that your saddle would probably be more comfortable with the nose up. Beautiful bike too, and I am jealous as mine is rough. Preliminary ride was positive with feel and hopefully yours is the same. Seat position is really important.