Thank you all for some great references! I've been scouring (and devouring this info) over the past couple days. I especially enjoy reading the Brazer Archives.
I basically got into proofs when, because of limited budget, wanted examples of 19th century US classics. Prices of proofs were within range and you get "proof-life" engraving impressions (couldn't resist the pun). I especially like the 153P3 and 76P4's below. I have a 153 used but, as with most real used examples, the purple is washed out and cannon/rifles detail is very faded ... the proof is crystal clear.