I recently won a Summitar (ca.1949) in really nice condition. It didn't come with a hood, but I wanted to protect the lens' glass which I understand is quite soft and also prone to flare. The SOOPD's large size does not appeal to me.
So here's what I did. I found a SNHOO which is Leica's A36 to E39 adapter ring. It was not cheap, but still less expensive than the usual alternative Leica-brand hoods (IROOA or SOOPD).
Using the SHNOO adapter allows me to use an after-market E39 screw-in hood, and optionally with the E39 filters I already own in my kit. I can't say for sure how effective this setup is in preventing flare, but I will certainly be testing it out.
I'd like to add that my Leica 12538 plastic hood (for 50mm M-Summicron) did not fit securely onto the Summitar or the SNHOO. I've also read elsewhere that
some 12585 hoods also do not fit so well on
some Summitars. YMMV.
Here's Ritz Collectibles' website for A36 filters, hoods, adapters, etc:
http://www.ritzcam.com/catalog/index.php
Search eBay seller "heavystar" who sells new, after-market, screw-in hoods. The one I bought was metal, round, and vented. I've no connection with these two businesses, except to be a satisfied customer.
good luck!
rt