What should the constructor for a record look like?

[HiPhish]

Hello, it's me again, the guy who wants to implement MessagePack [1] in Guile. 
The specification defines a type of "extension" [2], a pair of an 8-bit 
integer and a byte array for data. Implementing this type as a record is 
obvious, but what should be the name of the constructor?

    (define-record-type ext
      (make-ext type data)
      ext?
      (type ext-type)
      (data ext-data))

Either `make-ext` or just `ext` seem appropriate. I have seen both types of 
constructors, what do you guys say? And while I'm on the topic, what about 
types? An extension is only valid if the first field (type) is an integer from 
-128 to 127 and the data a vector of bytes (integer in the range from 0 to 
255). How do I enforce this invariant without static typing?

[1] https://msgpack.org/
[2] https://github.com/msgpack/msgpack/blob/master/spec.md#ext-format-family