3rd Rail and most other brass engine makers use smaller screws than what you you usually find in die cast tenders. The ones to look for for will be in the 4 corners of the underside. There may be more along the side in the middle depending if its a longer tender like a SF Northern. Maybe one pair or rarely two extra pairs of screws.
If you are still not sure then turn the tender over and take a picture and post it here.
Pete
