Your oblong pierce-cut edge oblong platter was made by International Silver Co and Meriden of Meriden Connecticut was one of the subsidiary members.
The S.P. stands for "silver plate". Plated silver does not have a precious metal melt down value like sterling silver.
The high value of your large platter is $45 - $55 on today's market.
Just checking in to see if you viewed your answer and if you are seeking further assistance?
Please let me know.