If the trim is 18V those ID plates were originally on a HX XX7 Sandman van. Numbers all fit together fine so the plates almost certainly belong together.

It is a consecutive PSN with a few others I have recorded around it:

L371976 (this one)
L371977
L371978
L371980

They appear to be right on the changeover from 1/77 to 2/77 compliance. L371976 and 77 are 2/77 compliance and the other two are 1/77. The chassis numbers of them reflect this too and L37196 has the highest chassis number of the four.