Parcelpostguy:Chapter 25 in the
International Rate Book was incomplete.
In the link Don posted in the second post of this thread:
https://stampsmarter.org/learning/H...yWRates.htmlThere is a link to his
International Rate Update, a 94 page update taking the rates for many table forward to 2014.
The
Recorded Delivery Fee Update is here:

From this table, the Recorded Delivery Fee for your cover was $1.00.
Determining the Airmail letter rate for 1993 was more difficult. In the Appendix I, page 365, is a continuation of the Table:
Simplified Domestic and International Rate & Fee Chart, Great American Series Period. Towards the end of that chart, there is an entry for"Airmail Letter to all countries Except Can/Mex, > 1/2 oz, < 1.0 oz, which shows the rate as of Feb 3, 1991 was 50 cents. This rate was effective until July 8, 1995, when it increased to 60 cents.

Thus the $1.50 postage on your cover was correct.
Q.E.D.