pickenatit..was reading your post about the single 15 cab....i believe that it may have been introduced in 1966 as the , as the amp was called, the sonic 1 with a single 15 cab with a jbl d-130 speaker...the single d-140 was an option or choice for that amp head..now, that cab was made with the rear loading speaker. the back came off. that's how they got it in the cab, not being smart here...now, that cab was made right up until 1970. rear loading speaker. now, by 1968 , the d-130 speaker wasn't offered in the cab any more..just the d-140. ok ? when 1971 rolled around the cab , along with sort of a new sunn company, not the conrad company, made the single 15 cab, front loading. the back of the cab was sealed...and the porting was different on the inside. instead of the folded horn, if you will, facing up at a 45 degree angle, it faced down. the cab on the inside was completely different from the old style cab..the jbl's weren't offered anymore...so...if the back of your cab comes off...you have yourself a gem which very few own to this day...i have two....they sound great....when you get older. rolling two single cabs around and stacking them is better than a 200s cab or a 2000s ...your back will dictate which cab you will use that night for your gig...hope this info helps...Bob from mass.