Regarding the 'vator assembly instructions on this page,
I believe that the rivet callout in Step 12 might be a misprint, since the MK419BS is a countersunk rivet.
The same goes for Step 15, which calls for MK421BS.
This drawing shows those rivets as MD419BS and MD424BS, respectively: http://www.soaridaho.com/Schreder/Schreder/Soaring_April_1976_Page_34.JPG
And, by the way MD419BS is an old USM Pop rivet number. It means:
M = Monel rivet
D = Dome head (or K= "Kountersunk")
4 = Nominal dia in 1/32"ths (but 3 = 7/64", go figure)
1 = Measure of length, 1st digit
9 = Measure of length, 2nd digit
B = Steel stem (I think)
S = Steel stem
The modern equivalent for the MD419BS that you're most likely to encounter is the Cherry MSP-42 commercial rivet, which Aircraft Spruce seems to have in stock. It has a slightly longer grip length, but that just leaves a little more rivet inside your airplane. No big deal. The Cherrys have a slightly different numbering convention:
M = Monel
S = Steel stem
P = Protruding head (or C= countersunk)
4 = Nominal dia in 1/32"ths (or 3 = 7/64")
2 = Measure of length
At least, I'm pretty sure that's the way it works.
This table at the Aircraft Spruce Web site shows the nominal equivalents between the USM pops and the Cherry commercial pop rivets (and their prices):
For rivets not on that table, like MD421BS, choose a Cherry MSP or MSC commercial equivalent based on the grip length. And by the way, I've got MD419BS and MK419BS, and probably MD424BS, MD421BS, and MK424BS in stock if you need them.
Editor's note: You can contact Bob at firstname.lastname@example.org