No oyu haven't missed anything. A PQR for 316 can be used to weld 304 if the WPS is written in a manner that makes it clear what materials you are welding to. You can do it two ways. You can write another WPS for 304/308 or create a seperate data sheet that lists all the individual 300 series SS alloys and the fillers to be used and tie it in to your WPS, and list on your WPS 3XX for filler metal classification. Note, this is not a code requirement but is sound engineering practice and most likely required by your customer. If they know what they are doing.
Right on the button, as usual js55. When I write a similar WPS, often times I will simply write P8 to P8 and then a typical set, such as the 300 series, as you mention.
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