Mead#10 Envelopes, Gummed Closure, All-Purpose 20-Ib Paper, 4-1/8" x 9-1/2", Plain White, 50/Box (75050)

Which measurements are these envelopes (75050)? The product title says 4-1/8" x 9-1/2", but the product description says 4-1/4" x 9-1/2".

Why, at checkout, does this indicate these envelopes have a california's proposition 65 warning?

Are these security envelopes with the pattern inside so someone can’t see in?

what is the designated paper weight (e.g., pound) for these envelopes?