In Vietnam, some battalion level signs were made for HQ areas, but nothing was official (see photo below).
This is the official 26th Regiment crest. I receive a lot of requests for where to get battalion level patches - e.g., 1/26 patch, 2/26 patch, 3/26 patch, etc. Fact is, no such thing exists - not officially, anyway. During WWII, the 5th Marine Division created its division seal. Each regiment (26th, 27th, and the 28th) emblazened the regimental number over the division number. Known as the "Spearhead Division", a blue spearhead is embedded behind the golden "V". Thus establishing our regimental patch you see here. During WWII, all marines wore a division patch on their shoulder. Prior to WWII a small regimental command structure was always in garrison at Camp Pendleton, CA, headquarters of the 5th Marine Division. These were professional marines, hence the moniker "The Professionals".
Anything else you may find out there, such as on Sgt. Grit's site is BOGUS, and was created recently (I even spoke with the woman who created a patch to cover all three battalions). The official regimental patch is the one you see here. In our regiment, battalion levels did not have patches, they only had Guideon flags (much like the platoon Guideon you saw in boot camp).
Proof of this can be seen in the 1968 photo below, taken on Okinawa. There you can see the official regimental logo of the 26th Regimental Landing Team of the 5th Marine Division. Further proof can be found in WWII photos.
Hope this clears things up for everyone.