I recently purchased a Sweetheart whistle from a fellow chiffer and I was just wondering if anyone can tell me when it was made. Has the S in heart logo on front. Back side of whistle body and head are both marked " M 1958". And of course body is Marked “D”. Said to be made from black walnut. Joint has what looks like red felt. Seems to require a little more wind than a Sweetheart I owned previously Seems like a very nice whistle. Very satisfied with purchase and seller was super quick in shipping. 
Does it matter if this whistle is left together or should the head and body be separated when not being played? 
Whistling Pops, I’m not aware of any listings anywhere about dating a Sweet instrument based on the serial number. Maybe you could contact the folks at Sweetheart and ask nicely if they could tell you when it was made. Wouldn’t it be cute if it was made in March of 1958? But I think they didn’t start making instruments until maybe the 1970’s, ICBW. I’m not at all sure when wooden whistle production began.
I’ve had a couple Sweetheart whistles in the past and I recall the serial numbers on mine being higher than yours, 3000-ish (could have been 300-ish memory being what it is) and bought in the late 90’s. Maybe we could zero in on a rough date if folks here have Sweet whistles with closer numbers to yours. The serial numbers on mine had no letters just the numbers if I am remembering correctly. And mine were bears to play above the second octave D with lots of push and air velocity needed to get to the high a and b. Mine had red thread (cotton?) on the tenons but yours it could have frayed and matted to look a bit like felt.
You might want to dis-assemble the wooden whistle after playing and swab out the bore too. And it’s a good idea to oil the whistle occasionally to keep it in good shape. Sweetheart has care instructions on their site.
Hope that helps.
Feadoggie
Write to the nice people at Sweetheart and ask if they can date your whistle.
Yes, by all means store the whistle in two pieces. Otherwise the red felt or whatever to mat down.
I’ve got an older blackwood one with the tulip shaped head marked M1779. Somewhere in my “stuff” I’ve got a hand written note from Ralph explaining what that means. I’ll look for it today.
I spoke with him on the phone shortly after I bought my whistle on ebay and the red wrap is 5 ply wool yarn. He recommends something like Chapstick for a lubricant.
I can send you enough to last a long time if you want. I keep mine joined together, keeps the joint clean and I can always replace the yarn or add more if necessary.
Jude
ps
I found the note from Ralph Sweet in my “stuff” and the M1779 means 11,779. He states that my whistle was made August 2002.
“M” is for myriad = 10,000. M1779 = 11,779.
Heyjude, thanks for the info! 
I also have the same whistle and it looks like it’s made from Rosewood. And on the back it says M 671. And I always keep it together to protect the red yarn. It’s a strong playing whistle especially in the second octave.