I also have the book referenced by the OP back in 2009 but in Czech. Per the Zrození Legendy book by Pazdera-Skramoušský (2005), a rough translation from Czech appears to indicate that 5 acquisition patterns/samples and validation series (akviziční vzorky a ověřovací série) were produced in 1975 with serial numbers 00001 to 00005. The one in the video matches the 00001 serial number. These were used for testing functional reliability and accuracy among several criteria. E.g., the book states that in June 1975, ser. no. 00003 was used in a 5,000 rounds test. There were questions with accuracy and was considered the biggest weakness of the design at the time.
In another book by the same co-authors, it states that in March 1976 the "ČZ model 75" was approved and released for series production, which officially commenced on 1 June 1977.
Unfortunately, no information on the serial number scheme beyond the first five samples is provided and also no data on what the pistols beyond these first five were used for before approval for serial production. Based on the serial numbers on both of your 1977 pistols that you have both graciously supplied, one might be tempted to think it possibly started with 10000 or 11000 so there would be a gap of unused serial numbers. Also, there would be gaps (possibly for control purposes) between acquisition patterns/samples and future patterns as the pistol was developed for serial production approval in March 1976. All the more fascinating with these gaps in information!
There is a photo in the books of ser. no. 01010 from 1975 that is a mere few numbers away from Clausewitz's no. 01002. The photo is b/w so it's difficult to compare the finishes (it almost looks brushed) between the two. Thanks for sharing the photos and updating this old thread.
Edit - one other note regarding serial numbers, there is a photo of prototype no. 002 in the book which appears to bolster the case for jumping around with serial number ranges for specific pistols during this period.