Nothing has really changed since 1960 as far as the fitting instructions go.
The mask is almost the same. Maybe a few changes to the type of rubber or silicone used in the mask. Just remember make sure your mask fits the purpose it is intended for.
So this is the one I have in my survival kit. Its simple and the filters keep most gases from entering.
Israeli Rubber Respirator Mask NBC Protection For Industrial Use, Chemical Handling, Painting, Welding, Prepping
Vktech Industrial Gas Chemical Anti-Dust Respirator Mask Goggles Set (Style A)
CDR 4500 Elite Gas Mask For Nuclear , Biological & Chemical Warfare NBC Protection Military Grade US NIOSH Certified Survival Full Face Mask For Kids Adults, Comfortable Robust Design
$19.99 I want to get a pair of these and put some good filters on them for work in the yard.
JOLIN Dual Anti-Dust Spray Paint Industrial Chemical Gas Respirator Mask & Glasses Set
3M 07193 Dual Cartridge Respirator Assembly,Organic Vapor/P95,Large
If you only need protection from the fallout of the ash . Then here are a few good one’s.
Anti Pollution Face Mask Eco Gear with Military Grade Protection | Anti Smoke, Exhaust Gas, Dust, Pollen Allergens | Hiking, Running, Walking, Cycling, Ski and other Outdoor Activities (Pure Black)
Unigear Activated Carbon Dustproof/Dust Mask – with Extra Filter Cotton Sheet and Valves for Exhaust Gas, Pollen Allergy, PM2.5, Running, Cycling, Outdoor Activities
Fresh Air Supply Smart Electric Mask Air Purifying Mask Anti Pollution Mask N95 for Exhaust Gas, Pollen Allergy, PM2.5, Running, Cycling and Outdoor Activities
NATO Israeli NBC 40mm Gas Mask Filter Only!
Gas Mask Filter 40mm CBRN Military Grade Gas Mask Filter Made in 2017