Hello, this spoon being silver plated will be difficult to close date. The date letters used on solid silver do not apply for silver plate items. Some of the very large electroplate manufacturers did however have "systems" of their own that have been near fully or partially interpreted. Hallmarks, are the marks you find on solid silver, and among those there will usually be a silver guarantee mark, maybe numerical, pictorial or written. As examples, many continental spoons have 800 on to guarantee 800 parts silver per 1000 parts. Most 18th century onwards English silver will have a standing lion on to guarantee 925 parts silver per 1000 parts. This is also known as the "Sterling" standard of silver and some items do just have that written mark on them. Your written marks should be better called "manufacturers marks". Could the crest on your spoon be a complicated monogram?
Your spoon is still an interesting spoon. The "MR&S" may be the initials of the manufacturers of the spoon and the mark has been seen before but that manufacturer is still, I think, to be determined. "&S" almost invariably goes with and son or sons.
The "IA" mark is likely an arbitrary value of the electroplating quality. The mark is usually written "A1". The pictorial mark with the cross keys in the shield, is possibly significant though. Henry Wilkinson's of Sheffield's cutlery concern was known to use the cross keys in their manufacturing marks.
Henry Wilkinson could have made your spoon, but possibly for a retailer or cutlery dealer, whose initials were "MR&S". Regarding the "Y" mark, Henry Wilkinson & Co Ltd were bought out in 1892 by Walker & Hall who did have a form of dating system. We have an "x" on the example that I had saved on my laptop.
Let’s see what any others think.
Fishless