As in "FOO.TXT;" ?
That certainly *used* to be a valid filename, back in V4/V5 days anyway,
  That's legal.   The longer story is that version numbers start with 1, and
";0" is shorthand for the most recent version.   "FOO.TXT;" is just
shorthand
for "FOO.TXT;0". 
  It's also legal to say "FOO.TXT;-1" meaning the previous version (i.e.
most recent -1), as is ";-2", ";-3", etc.
  And, FWIW, it's also legal to use angle brackets in VMS path
specifications (e.g. "DISK:<DIR.DIR.DIR>FOO.TXT").   Both the use of
angle
brackets and using a second "." in place of the semicolon (e.g.
"FOO.TXT.1")
were put into VMS long ago as an aid to ex-TOPS20 users.
Bob