I had the 60" version, I always used off the shelf unmarked bolts, worked great. Broke LOTS of them!
Yes, the unmarked bolts are grade 2, 3 dashes = grade 5, and 6 dashes = grade 8. I found it very difficult to find 5/16" grade 2's for my three point snowblower at local farm equipment dealers, big box hardware stores, and local bolt specialty shops. I finally found them online at a wholesaler and through their dealer list got them at the one local farm equipment store I had not checked. Part of the difficulty was the 5/16" size that I gather is relatively uncommon as opposed to say, 1/2".
My local big box stores such as Rona, Home Depot, Cdn Tire, Home Hardware, mainly sell grade 5 and perhaps some grade 8's. Even the local fastener stores(BC Fasteners, Bolt Supply in Calgary, Acklands) here don't seem to carry grade 2's.
Here's a table showing shear strengths:
View attachment Bolt_Shear_Strength.pdf
The 1/2" grade 2 bolt the OP mentioned has a shear strength of 8,717.5 lbs. while a grade 5 bolt's shear strength is 14,136.5 lbs.
Last year my neighbour "temporarily" used a grade 5, 5/16" bolt in a blower like mine instead of a grade 2. The result was a bent auger after picking up a rock. He is a machinist and was able to fix the auger on his press.
The main point is to use the size and grade shear bolt the manufacturer specifies.