the first three numbers - 441 - indicate the model no. 400 would be a series 1 2000 manual. 436 a series 2 2000 manual. 441 is, of course, a 2000 Auto series 2. These numbers go up to 869 for a Nigerian markeet 2200SC, of which 100 were built in the '74 model year.
The remaining numbers are then the commission no, in rough terms, the build no within that model sequence. Starting at 00001 and climbing in rough date order. Note that cars are not necessarily built in sequence. If some awkward options were ordered, the commission no would have been generated in date order but perhaps not built for a while until all the optional parts had been drawn together. This is particularely true at the very end of production in late '76 /early '77 when commission no's are all over the place in terms of date sequence.
The final letter, in your case B, indicates the spec status of the car. For instance the change from an 11AC alternator to an 18 ACR is accompanied by a different wiring loom and other changes, so the suffix letter advances by one. As yours is a B, that is exactly the change that has taken from place from a suffix A. Sometimes the change of suffix letter is much more significant. On a series 1 V8 (425 model number) the change from suffix A to suffix B comes with different spring and damper specs, a different diff and revised bush sizes in the rear trailing arms.
The best reference to fully understand all of this is the James Taylor book on the P6 that Nick Dunning and I assisted him with updating recently.
If you would like to see the factory record book containing your car - which will tell you what day it was built (ie when it went down the line0, what colour it was originally and when it was despatched from the factory to what dealer - you can apply to Gaydon for a reading room ticket. Then you will be able to book a week in advance to visit and exmine the actual factory record ledger in which this is recorded. But you won't be able to take a copy of it, because Gaydon would like to sell you a posh certificate that contains this information.
Chris York